I honestly don't remember when I clicked the update button, but I don't think this should be relevant because after syncthing stopped working I uninstalled it and reinstalled it from scratch.I'am just wondering if possibly you clicked the update button, whilst this was still running? Maybe this corrupted the database while the update was taking place.
If you can remember anything else from the issue you experienced, or can replicate it again, let me know. Aside from that, i'am at a bit of a loss and again edging towards a possible bug with Syncthing, or, unstable PSU.
Once reinstalled I DID NOT update, but after rebooting, syncthing stopped working anyway.
I can try again using a new DietPi installation on a different SD card to see if this changes anything.
I'll report back.