I just wanted to add that I came across the same thing. I’m not sure if it’s intended behaviour or not.
I shared a folder from machine A to machines B, C and D. In the “add folder” window on machines B, C and D, the option to save the new folder was not available if trash can file versioning was selected. The “save” button was grayed out and not selectable, as described above. With other file versioning methods the “save” button was selectable.
After saving the folder with “no file versioning” selected and then editing the folder properties again, it was then possible to save the folder with “trash can file versioning” selected.
This occurred whether I was adding the folder from the notification prompt or directly using the folder id. The 4 machines I’m using are running windows and android with a mixture of syncthing versions v1.7.1 and v1.8.0