This happened with version 4.3.1 on a Linux system: I added a few dozen new entries, saving the database after each new added entry, when suddenly the GUI told me that the database has changed on disk and forced me to review the changes, showing me the changes I have just made myself!
I am pretty sure no other process has actually updated the database on disk.
I know that there is nothing to review and I do not care if for some reasons the file modification date is not what jabref expects, I just want it to save and override the file on disk, but Jabref apparentlly does not allow that. This is very annoying.
So what I have to do is kill/abort jabref (losing the entry I was in the process of adding) and start it again.