Cannot Adjust Mic Input on Garageband

I'm trying to make Audio recordings using the iPhone built-in microphone.

I choose "Audio Recorder" with setting "Clean".

The recording input level slider on the left-hand side of the screen is STUCK at about the 65% level (cannot move it up or down).

I click on the "Microphone" icon next to this slider, there's NO option there either to set the input level, I only see "Inter-app Audio" and "Noise gate" options.

There's also no entry from Garageband in the iPhone Privacy->Microphone settings to allow/disallow access to the Microphone.

The microphone input is clearly working (can see the sound level meter lighting up when I produce sounds), but I'm unable to adjust the input level gain...

Any ideas how to fix this?