image conversion to webp sometimes resuts in smaller image with black background

Just a wild guess could it be that your images have saved clipping paths inside them Photoshop can save them with the jpeg ... can't not find one in the Uploaded original... but that could be cleaned up by metadata removal during upload..

just a guess

I dont think so it also happens on full pictures as well.
Canva is used for the composites.

still an issue.

sourses are .png