If you open the web UI, go to the About screen and then the Paths tab it will tell you where the log file is, if there is one. Without -verbose, -audit or any debug options it’s unfortunately not likely to tell you much.
I have been having similar problems which are most likely down to some interaction between Syncthing and Android.
I have a large knowledgebase (16000+text files) which I sync between multiple systems, Linux & windows and a couple of older Androids (9 and earlier). I set up a new Android 13 system with that directory set to receive-only. At some point I recall a message saying something about Android making it read only, and the receive-only in Syncthing got flipped to send-only with a local state of 0 files. It then went about propagating this state to the other systems. Fortunately I spotted it early and was able to disconnect it from the network (and delete the syncthing config for that folder on the android system), then recovered the full state from a machine that was offline when the damage happened.
I have had that ‘flip to read only’ twice now on that android system and am very wary about re-enabling sync on that directory.
The directory is physically internal on the Android device, NOT on an SD card. Android shows it as /storage/emulated/0/wiki
I am keen to find out what is going on. There is nothing in the Log File section of the Paths tab. How do I enable logging on Android?