I do have the same problem - I assume the other MIDI plugins are AUv2, not AUv3.
Additionally, I have no idea how to debug it since no breakpoint is being hit and I have the slight feeling that the callbacks aren't even called.
The not-at-all-present documentation makes it even more difficult.
Yes - I emailed one developer - their plugin is indeed Auv2
This is very frustrating I must say. Like you say the documentation _really_ doesn't help