I finally debugged and learnt that sampling rate conversion with new iOS 18.0 is failing for my audio unit. Fixed by handling correcting sampling rate.

I confirmed the behavior. This works with 17.x but fails with 18.x iOS versions. Some bug introduced. installTap is only called around 10 times with buffersize of 4096 and then never gets called back again. Only happening it seems with Bluetooth Speaker connected and iPhone's microphones are input.