It seems to me that there is a huge bug in group folders. If user A share a group folder with user B (out of the group members), user B could upload files, but as soon as he leaves the folder, the file “disappears”. Even user A cannot see it. The file is still there (trying to re-upload it will generate an overwrite permission request) but is virtually invisible.
The share via link should be working, but not direct share.
This is a HUGE bug.