AirPods 4 clicking sound and stuck on 1-channel audio (HFP lock) on Mac Mini

Hardware: Mac Mini M4 running 26.2 (25C56)

AirPods: AirPods 4 (Firmware 8B21)

Issue Description:

I am experiencing a persistent clicking/popping sound in my AirPods 4 when using them with my Mac Mini. The audio quality drops significantly, sounding robotic or "tinny".

Technical Observations:

When this happens, I checked Audio MIDI Setup and found that the AirPods are stuck on "1 ch 16-bit Float" (HFP/SCO mode) and I cannot manually change them back to "2 ch 32-bit Float" (AAC). The format dropdown is often greyed out.

Workaround:

The clicking only stops if I manually change the Sound Input in System Settings to the "External Microphone". However, macOS frequently defaults the input back to the AirPods, triggering the clicking sound and the 1-channel quality drop again.

Steps Taken:

Updated AirPods to latest firmware (8B21).

Reset Bluetooth module and re-paired AirPods.

Tried creating an Aggregate Device in MIDI Setup, but the system still switches back.

Temporary fix found by running sudo killall coreaudiod in Terminal, but the issue eventually returns.

Is this a known firmware bug with the AirPods 4 or a macOS audio handling issue? Any permanent fix to prevent the Mac from hijacking the AirPods microphone would be appreciated.