I just used gThumb recently to try to post something I had inverted, but posting it to sopuli/lemmy scrubbs the exif data and it was posted as if I didn’t do anything.
Yes, uploading to Mastodon and Lemmy will likely automatically wipe EXIF data by those sites.
So, gThumb uses exif data to modify pictures which is likely what is added.
*What alternative are you using?
For image cropping I've switched to Gwenview which I'm pleased with.