Solution for missing variations during export

As far as I can understand, your plugin creates an array with the ajax action wp_ajax_wsm_get_products_or_export and puts all the products and variations in this array. The problem is your plugin overwrites the added variations and products because every iterations starts using the offset as the next id, even if the last iteration sets the last id farther.

To solve it I modified your plugin: the ajax call now requires the item variable, and the json answer returns the last ID added in the array.

woocommerce-stock-manager.php

add_action( 'wp_ajax_wsm_get_products_or_export', 'wsm_get_products_or_export' ); 
    function wsm_get_products_or_export(){
    
        $jump = 10;

        $i = sanitize_text_field( $_POST['item'] );
        $offset = sanitize_text_field( $_POST['offset'] );
        $posts_per_page = $jump;
        $args = array(
            'post_type'      => 'product',
            'posts_status'   => 'publish',
            'posts_per_page' => $posts_per_page,
            'offset'         => $offset
        );
        $_products = new WP_Query( $args );
        if( !empty( $_products->posts ) ){
            $data = array();
            $i = 1 + $i;
            foreach( $_products->posts as $item ){

                $product = wc_get_product( $item->ID );
                if( !empty( $product->get_sku() ) ){ $sku = $product->get_sku(); }else{ $sku = ''; }
                $product_name = $item->post_title;
                if( !empty( $product->get_manage_stock() ) ){ $manage_stock = $product->get_manage_stock(); }else{ $manage_stock = ''; }
                if( !empty( $product->get_stock_status() ) ){ $stock_status = $product->get_stock_status(); }else{ $stock_status = ''; }
                if( !empty( $product->get_backorders() ) ){ $backorders = $product->get_backorders(); }else{ $backorders = ''; }
                if( !empty( $product->get_stock_quantity() ) ){ $stock = $product->get_stock_quantity(); }else{ $stock = ''; }
                $product_type = $product->get_type();

                $data[$i]['id']   = $item->ID; 
                $data[$i]['sku']          = $sku; 
                $data[$i]['product_name'] = $product_name; 
                $data[$i]['manage_stock'] = $manage_stock; 
                $data[$i]['stock_status'] = $stock_status; 
                $data[$i]['backorders']   = $backorders; 
                $data[$i]['stock']        = $stock; 
                $data[$i]['type']         = $product_type; 
                $data[$i]['parent_id']    = ''; 
                
                $i++;

                if($product_type == 'variable'){

                    $args = array(
                        'post_parent' => $item->ID,
                        'post_type'   => 'product_variation', 
                        'numberposts' => -1,
                        'post_status' => 'publish' 
                    ); 
                    $variations_array = get_children( $args );
                    foreach($variations_array as $vars){

                        $item_product = wc_get_product($vars->ID);
                        if( !empty( $item_product->get_sku() ) ){ $sku = $item_product->get_sku(); }else{ $sku = ''; }
                        
                        $product_name = '';
                        foreach($item_product->variation_data as $k => $v){ 
                            $tag = get_term_by('slug', $v, str_replace('attribute_','',$k));
                            if($tag == false ){
                                $product_name .= $v.' ';
                            }else{
                                if(is_array($tag)){
                                    $product_name .= $tag['name'].' ';
                                }else{
                                    $product_name .= $tag->name.' ';
                                }
                            }
                        }

                        if( !empty( $item_product->get_manage_stock() ) ){ $manage_stock = $item_product->get_manage_stock(); }else{ $manage_stock = ''; }
                        if( !empty( $item_product->get_stock_status() ) ){ $stock_status = $item_product->get_stock_status(); }else{ $stock_status = ''; }
                        if( !empty( $item_product->get_backorders() ) ){ $backorders = $item_product->get_backorders(); }else{ $backorders = ''; }
                        if( !empty( $item_product->get_stock_quantity() ) ){ $stock = $item_product->get_stock_quantity(); }else{ $stock = ''; }
                        $product_type = 'product-variant';

                        $data[$i]['id']   = ''; 
                        $data[$i]['sku']          = $sku; 
                        $data[$i]['product_name'] = $product_name; 
                        $data[$i]['manage_stock'] = $manage_stock; 
                        $data[$i]['stock_status'] = $stock_status; 
                        $data[$i]['backorders']   = $backorders; 
                        $data[$i]['stock']        = $stock; 
                        $data[$i]['type']         = $product_type; 
                        $data[$i]['parent_id']    = $item->ID; 
                
                        $i++;

                    }
                }

            }
            $offset = $offset + $jump;
            //$data = json_encode( $data );
            $reponse = array(
                'status'    => 'continue',
                'data'      => $data,
                'offset'    => $offset,
                'item'      => $i-1
            );
            echo json_encode( $reponse );
            exit();

        }else{

            $reponse = array(
                'status'    => 'finish',
                'text'      => __( 'All done! Close.', 'stock-manager' )
            );
            echo json_encode( $reponse );
            exit();

        }

    }

admin/views/import-export.php

jQuery('.product-export').on( 'click', function(e){  

        e.preventDefault();
        jQuery( '.export-output' ).empty();
        jQuery( '#csv' ).append( 'Generating csv file' );
        jQuery( '#csv' ).css( 'display','block' );

        var offset = '0';
        var item = '0';
        wsm_export_products( offset, item );        

        function wsm_export_products( offset, item ) {
            
            var data = {
                'action'  : 'wsm_get_products_or_export',
                'offset'  : offset,
                'item'    : item
            };

            jQuery.post(ajaxurl, data, function( response ) {
                
                var result = jQuery.parseJSON( response );
                
                if( result.status != 'finish' ){
                    
                    var jsonObject = JSON.stringify(result.data);

                    jsonObject = jsonObject.slice( 1 );
                    jsonObject = jsonObject.slice(0, -1);

                    jQuery( '.export-output' ).append( jsonObject + ',' );

                    wsm_export_products( result.offset, result.item );
                    

                }else{
                    
                    var string = jQuery( '.export-output' ).text();
                    string = string.slice(0, -1);
                    
                    string = '{' + string + '}';

                    jQuery( '#csv' ).empty();
                    jQuery( '#csv' ).append( 'All done!' );

                    var data = {
                        'action'  : 'wsm_get_csv_file',
                        'data'   : string
                    };

                    jQuery.post(ajaxurl, data, function( response ) {

                        var data, filename, link;
                        var csv = convertArrayOfObjectsToCSV({
                            datas: JSON.parse( response )
                        });
                        if (csv == null) return;

                        filename = 'stock-manager-export.csv';

                        if (!csv.match(/^data:text\/csv/i)) {
                            csv = 'data:text/csv;charset=utf-8,' + csv;
                        }
                        data = encodeURI(csv);

                        link = document.createElement('a');
                        link.setAttribute('href', data);
                        link.setAttribute('download', filename);
                        link.click();

                    });
            
                }
    

            });
        }
    });

Hope this will help.