In the progress of the upcoming Cubasis update the topic has been evaluated by our engineers with the following outcome:
The IAA apps that are not listed as instruments (only for audio track inputs) are in fact IAA generators (= only for audio tracks) and not IAA instruments, i.e. they cannot receive MIDI via IAA (only via CoreMIDI).
In summary, everything works as intended and no fix will be possible.
Best,
Lars