If you mean the web interface, that synced perfectly with the Android app for me for years, but it seemed like it stopped syncing properly sometime last month? (Is there a proper desktop app? I'd prefer that to the using my browser.)

I've had good success with the beta web client

It does for me, but not continuously flawless. I sometimes have to close and re-open the mobile app before it skips to where I left off on the desktop app.

If the owner of the code is publishing my the app, there is no problem as they just own the code and aren’t using it under any license and aren’t bound by the GPL. Similarly an owner of GPL code can relicense it for use on the App Store. Not ideal, but it can be workable.