Hello. Long time User of Music Studio since 2011/2012.
Same Issue here. The main Problem does seem to be iOS 13, As Music Studio worked perfectly on My iPhone 6 Plus and iPhone X Max. Once I had Updated, The Application now opens In a very distorted Interface as I am sure many of Us are aware of by now. The Play, Record, Pause Buttons are hovering over an extremely enlarged Keyboard and cannot be Interacted with. The Tabs for Keyboard, Instrument, Track etc. are forced to the upper left of the Screen and also are unable to be Interacted with.
I can only wonder how much Change that iOS 13 has done to the System that could result In such a drastic Issue to appear. Hope that We can get an ETA on when a Patch may become available. Really need to get back to using the App for My Music as I chart out all My Tracks on There. 
Thanks!