AirPods Max (USB-C) Head Detection breaks auto-switching

I bought a brand new pair of AirPods Max (USB-C) directly from Apple after returning a previous pair to Amazon due to the same issue, following advice from Apple Support. Initial setup worked perfectly and automatic switching between my iPhone and Mac functioned as expected.

Shortly after, auto-switching became inconsistent. The “Moved to…” pop-up stopped appearing reliably, audio routing did not always follow the active device, and switching between my iPhone and Mac became hit-or-miss.

After multiple factory resets and isolating variables, including turning Bluetooth off on other nearby devices, I discovered something interesting. When Automatic Head Detection is enabled, auto-switching becomes unreliable and device handoff feels inconsistent. When Automatic Head Detection is disabled, auto-switching works perfectly, the pop-up appears immediately, and device handoff is stable.

All devices are fully updated.

Has anyone else experienced this behaviour? Is this a known firmware issue or a conflict between wear-detection sensors and the automatic switching logic on the USB-C model?