I am getting this exact same issue. My iphone 11 running iOS 18.3. The device is working fine with the exception of all the music making apps I use which either crash, hang while loading or are unresponsive when loading. The common theme may well be that they all use MIDI. Apps: Loopy Pro Koala sampler AUM Synth One King of FM King of Digital Numa Player Poly 2 All other apps seem to be working fine. It seems that something has gone wrong with an API that all these apps use.

Update, fixed: I’ve just updated to iOS 18.3.1 and the issue is fixed; all my music making apps are now opening again.