Firstly, just to give more context, this is the affected Device A.
This is the other side, i.e. Device B.
Only 1 folder seemed to be affected by the issue. The folder itself is shared between multiple devices, but only the share between these two devices had the problem.
What is interesting is that the folder state was exactly the same on all devices.
However, the number of the stuck files was larger than the actual folder state.
This is interesting. The steps 1-5 did not help. However, after restarting A with
-reset-deltas, the issue seemed to have resolved itself for a moment, but then this came up.
New changes to the folder got stuck again, but this time it was not only A, but also other devices that had these files stuck trying to push them to B. Device B still marked everything as “Up to Date”.
Now, I am not a fan of resetting deltas/database on B, as the hardware is slow, and there are tons of folders, but there was no choice, so I did it, but then everything got quite messy. While the problematic folder shared between B and A seemed to get fixed, B itself got stuck trying to send stuff to other devices with no progress. I am not sure what the problem was exactly about, but I restarted Syncthing on B once again, and now it seems to be pushing indexes to the rest of the cluster slowly. It may take a few hours until everything stabilises.
I will report back again later once I can say for sure what the situation actually looks like.
Edit: The situation seems to have normalised, at least for now. I will write back if the problem re-occurs.