Thanks for the detailed report. I will try it myself to see if I can reproduce the issue. It seems strange that it affects a) jpegs and b) just from one camera and c) from one walk.
A few questions:
1) Can you import one of the problematic JPEGs onto the “Files.app” side side of the app and do similar steps to edit and export?
2) Would you willing to share an original image with me to try? If so, please email firstname.lastname@example.org with a full size original.
3) It looks like 14.2 has shipped, so perhaps it’s worth trying that? That’s what I’m going to install since it’s available.