Hi Drybonz,
Thank you for your message.
I gave the issue a short check on the iPhone X, and Cubasis 3 launches with the same orientation as Moog Model D, Model 15, dspMixFX and other apps.
Cubasis comes in landscape mode, since most of its components such as the sequencer/Arrange Window, instrument user interfaces, on-board keyboard are best-suited this way.
Best,
Lars