I am using researchxxl's fork now and it works fine, had 2 updates in the meanwhile
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Syncthing itself is fine. The unofficial Syncthing for Android was the problem.
You can get around the problem by installing Termux and using the official Syncthing client.
Using termux full time for running network facing services is pretty insecure. Termux should not be relied on for sensitive things, EVERY process runs as the same user.
There is also the android version of syncthing-tray
https://martchus.github.io/doc/syncthingtray/docs/android.html
https://martchus.github.io/syncthingtray/#downloads-section
https://martchus.github.io/doc/syncthingtray/docs/screenshots.html#mobile-ui-on-android
That dev has been around for a long time so seems like a decent option even if not as polished as syncthing-fork was.