Original image in webp format not uploading

I have the following issue: I use your plugin together with Webp Express which generates the webp versions of images, and it almost works perfectly. All custom image sizes & standard wp image sizes (thumbnail/medium/large) get created and offloaded in both their original file extension + .webp.

But as far as the original (full size), for that one only the file with the original file extension gets uploaded.

Do you have any ideas why this could happen?

I have searched your forum before posting this. Enabled WP_DEBUG and there is no error.