I have a PC connected to an Android device via Syncthing. However, the usual Android issues I have determined not to be an issue in my case: It is the PC that does not sync unless the web GUI is opened. When it is not, it appears to be disconnected from the phone’s side.
This may be related to the fact that this PC cannot access the web GUI unless it is accessed via syncthing.exe, as opposed to entering the URL directly. This issue I posted here: Unable to connect to web GUI without opening PC app