Syncthing is available as an addon to LibreELEC.
At the moment, the addon provides Syncthing version 0.12.24, which does not update itself automatically.
The addon will however be updated regularly to provide a current version of Syncthing.
If you wish to run the lastest version of Syncthing as soon as it is available (rather than when the addon is updated), you may enable automatic updates of Syncthing via the web interface (usually at libreelec.local:8384).
If you wish to keep your current version of Syncthing, disable automatic updates of the addon via Kodi settings (and make sure that automatic updates of Syncthing are disabled via the web interface).
Please provide you feedback about Syncthing on this thread.