When connect Yamaha keyboard to iPad with ipados 15.4.1 or 15.5 beta 1 or 15.5 beta 2 the music app likes korg module pro, cubasis, Camelot Pro crashes. I tried with other midi keyboard like Arturia or Native instruments and Midi works. Reading the forum seems somethink linked with Yamaha keyboard. Until ipados 15.4 all worked fine.
I have an old Yamaha YPG-635 and can confirm that MIDI apps no longer work with my iPad Pro 12.9" after installing 15.4.1. This is also affecting users of our app, Playground Sessions, which depends on system MIDI to follow user playing and provide feedback. As of beta 3 of iPadOS 15.5 (19F5062g), the bug has not been fixed.