Please check the Android app settings because you need to tick “run on boot” first to actually make Syncthing start automatically. If you still experience issues, then you should check your OS settings whether Syncthing is allowed to run in background (e.g. exclude it from battery optimisations, etc.). This varies depending on the specific device manufacturer and their Android flavour. You may want to check https://dontkillmyapp.com and see whether some instructions are applicable to your one as well.