I agree that is weird. I followed the steps you listed and could not replicate the problem.
However, I found another person reporting the same issue at https://feedback.photoshop.com/conversations/photoshop/photoshop-saving-copy-not-saving-over-jpg-file/5f5f46014b561a3d426de6cd
This is how they solved it, and I hope it works for you:
1) Close all Adobe related processes in activity monitor (Creative cloud, Core sync, CC library, services for adobe desktop app, adobe desktop service, Adobe IPC broker, AAM updater) 2) Navigate to following locations and assign the current user full permissions to adobe folders. To navigate click go from menu bar at then click goto.. type /Library/application support/ and click go. 3) To assign the permission right click on adobe folder and click Get info.. A property window will open. Then click on lock symbol to enable editing. Then click + sign to add your username. Then change read only to read and write. Then click gear symbol at the bottom and click apply to enclosed items and click ok.
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