The latter. SMAppService.agent(plistName:) returns a SMAppService instance. I can successfully call register on it (i.e. no error is thrown). However, the bundled LaunchAgent is not launched or the connection (I'm using the Swift API) cannot be established.

I am aware of that behavior and I'd even argue it is the "right" thing to do for the OS. However, I don't believe that is the problem here.