I am not sure whether this is something recent or not, but I do not think that I had encountered anything like it before v1.11.x.

How to reproduce:

1. Create an empty folder on Device 1.
2. Add 1000 small text files to the folder on Device 1.
   
3. Share the folder with 1000 files from Device 1 to Device 2.
4. Accept the folder on Device 2 and let it sync.
   
   
5. Delete the 1000 files from the folder on Device 2.
6. The two folders are stuck in the following state.
   
   

Please make sure to follow the steps exactly as I have written above. If the folder was already being shared between the two devices before adding the files to it, then the synchronisation is successful. I have only managed to make this happen when the files were added to the folder in the "Unshared" state.

Restarting Syncthing does make the folders finish the synchronisation.