Thanks for the high-res video. It does indeed appear that nothing is saved (not toast message, the title of the project in Spine stays at "Untitled*"). I've tried to reproduce this on my Mac with Spine 3.6.16-beta, but it works as intended (import data as new project, set image folder, save project as to Desktop).
Could you try the following:
- Save the project to a different location, e.g. Desktop
- Send us the Spine log file which you can find at Mac: <user home folder>/Library/Application Support/Spine/spine.log
Sorry for the troubles.