I haven’t upgraded to the latest version of iMS yet, so I can’t test, but I’m pretty sure the problem is as follows.
The latest update will include compatability with the latest Audiobus code. As this now relies upon IAA to stream audio between apps (due I believe to latest Apple security requirements in iOS8) apps automatically get IAA generator functionality as a byproduct. This allows IAA hosts to run these apps using IAA on audio tracks. This is not the same as fully including IAA instrument functions which make use of combined MIDI / audio tracks, where the track holds MIDI data to drive the app and freezing enables the audio to be captured.
The full instrument implementation allows the developer to include an IAA panel for switching back to and controlling basic host transport functions. I believe this can be done with IAA generators, but the developers must add this panel.
Korg haven’t implemented IAA, you just have it as a bonus due to their continued inclusion of Audiobus compatibility. If you check the latest update info on the App Store you will notice that IAA isn’t included in their list of new features.
If you need to sync start iMS with Cubasis, connect via Audiobus using an audio track and you can use the Audiobus panel. If you add a MIDI track and select iMS-20 Channel 1 as the MIDI output routing you can use MIDI continuous controller 111 to start and stop iMS at different points in the track. 127 to start, 0 to stop.