Since its creation, this folder at NAS was receive-only with ignore-delete, and its counterpart in the phone was “sent-only”. The synchronization was set up more than a year ago, and was used the way I described in the scenario 1.
That is, new photos were synced from the phone to NAS, and from time to time I moved the synced photos from the Syncthing folder at NAS to a photo folder. From the Syncthing perspective the files were deleted. This worked fine until yesterday, when I changed the unidirectional synchronization to bidirectional. Immediately, the Syncthing folder at NAS became out-of-sync with the problems we’re just talking about. In the phone, however, the folder looks synchronized.
Syncthing at NAS: v1.4.0, Linux (64 bit)
Syncthing at Android: v1.3.4
I’ll try to post the logs in the next post.