Set price filter not working

help! when I use the set_price filter for woocommerce the price changes for 2 seconds and reverts back to normal. please help.
my code is:

$args = array( 'post_type' => 'product',  'product_cat' => $key);
$loop = new WP_Query( $args );
while ( $loop->have_posts() ) : $loop->the_post(); 
    global $product; 
    update_post_meta($product->id, '_regular_price', (float)$value);
    update_post_meta($product->id, '_price', (float)$value);
endwhile; 
wp_reset_query();  

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