Hi Mateusz,
yes, conversion was not the problem.
I guess the main difference is, that your plugin writes into the “main” .htaccess where you also find WP’s own redirects as well as a long list of rules for W3 Total Cache.
The competitive plugin writes it’s own .htaccess into the uploads directory (and if you like into the themes directory).
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Rewrite rules were saved to the following files:
/is/htdocs/…/…/wp/wp-content/themes/.htaccess (themes)
/is/htdocs/…/…/wp/wp-content/uploads/.htaccess (uploads)
/is/htdocs/…/…/wp/wp-content/webp-express/webp-images/.htaccess (cache)
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So IMHO all the other redirects lead into the uploads directory and only then and there, the webp files are chosen.
I have deinstalled your plugin (sorry for that) so I cannot send you “your” .htaccess.
Try to install the other plugin to see which content their .htaccess has.
BR Heinz