file content become empty using plugin’s upload method

my greeting
our customer is using this plugin to import some of their post type content from their local env to our hosting provider. but the problem is the file become empty when using plugin’s upload feature even though the upload process completed and shown the correct file size. we did standard wp debug (deactivate all plugin and using default theme) but still no luck.
fyi wp media upload works normal, we can upload csv file and the file’s content is still there. the file deployed via ftp to ~\wp-content\uploads\wpallimport\files is working too. the only problem we found is when using plugin’s upload feature. we are using nginx and php-fpm to run the server but no log worth to mention from both of them.

may be we miss a place to check or you need any more information please let me know
thank you