These are the possibilities I tried here.
Cable
USB-B to USB-A with Lighting Camera Adapter or USB-B to Lightning (Agvee)
Devices
iPhone 13, iPhone 8 or iPad Air 2
iOS 15.3.1 work.
After iOS 15.4 don’t work.
Cable
USB-B to USB-A with USB-C Camera Adapter or USB-B to USB-C (Ugreen)
Device
iPad Air 4
iOS 15.3.1 works
After iOS 15.4 doesn't work.
MIDI DIN to USB-A with USB-C Camera Adapter or lightning Camera Adapter
All devices are working. Tried on iPhone 8, 12, iPad Air 2, 3 and 4. All with 15.4.1.
I'm using a DOREMiDi Brand cable. This is the only way I found to use SimplyPiano for while, at least, until Apple fix this problem.
More information here: https://discussions.apple.com/thread/253771323
