If possible, please create one topic per issue to make it easier for us to help you.
It is of great help for us, to have the steps to reproduce the issue following this bug reporting form.
Regarding Synthmaster
In your video it looks like, the connection between Synthmaster and Cubasis has disconnected.
Reloading the project should immediately solve the issue. We will try to reproduce the problem, and will exchange with KV331 Audio about it.
Please make sure to let them about the problem from your end at well.
Regarding KORG M1
Please specify the steps how you use KORG M1 with Cubasis (e.g. are external devices involved etc.).
Please use the bug reporting form, to allow us reproducing the issue.
It is not required, to send us various messages about the same topic.
One proper bug report per issue is sufficient and helpful for us.
Gave the Synthmaster One issue a short check with the latest available Synthmaster One TestFlight beta version and Cubasis 2.7 (iPad Pro 12.9", iOS 12.1), where things work as expected.
Steps
Launch Cubasis and create a MIDI track
Assign MIDI track to Synthmaster One AU via tapping its track icon
Played and record a few bars of MIDI
Switched presets
Changed user interface modes (normal screen, full screen, w keyboard, without keyboard)
Gave the issue a check and provided you with a download link to a short video of my steps via PM.
No need for MIDI thru or other settings when using iM1 IAA with Cubasis.
Please have a look at the clip and give it a try yourself.
One more thing regarding the black instrument rack window (which you have seen with Synthmaster):
As mentioned before, something like this appears when the connection with an external instrument is lost.
If you experience this case, you can give it a try to close/reopen the instrument rack.
Additionally, relaunching Cubasis should solve the issue.
In any case, please make sure to have KV331 Audio informed about the issue.