Comment 1 for bug 1949869

It looks like the rpi3 devices where this kernel is tested on uc18 fail to find the bluetooth controller. the pi-bluetooth snap normally takes care of the uart settings, but failed to run it's setup properly after refreshing to this kernel:
ubuntu@localhost:~$ sudo snap logs pi-bluetooth
2021-11-17T22:07:58Z pi-bluetooth.btuart[1422]: No controller attached
2021-11-17T22:07:58Z pi-bluetooth.btuart[1422]: Attaching Primary controller to /dev/ttyAMA0
2021-11-17T22:07:58Z systemd[1]: snap.pi-bluetooth.btuart.service: Main process exited, code=exited, status=1/FAILURE
2021-11-17T22:07:58Z systemd[1]: snap.pi-bluetooth.btuart.service: Failed with result 'exit-code'.
2021-11-17T22:07:58Z systemd[1]: snap.pi-bluetooth.btuart.service: Service hold-off time over, scheduling restart.
2021-11-17T22:07:58Z systemd[1]: snap.pi-bluetooth.btuart.service: Scheduled restart job, restart counter is at 5.
2021-11-17T22:07:58Z systemd[1]: Stopped Service for snap application pi-bluetooth.btuart.
2021-11-17T22:07:58Z systemd[1]: snap.pi-bluetooth.btuart.service: Start request repeated too quickly.
2021-11-17T22:07:58Z systemd[1]: snap.pi-bluetooth.btuart.service: Failed with result 'exit-code'.
2021-11-17T22:07:58Z systemd[1]: Failed to start Service for snap application pi-bluetooth.btuart.

Reverting to the previous stable kernel made it work again, and refreshing again makes it fail again, so it seems pretty easily reproducible. I noticed that rfkill with the working kernel shows both hci0 and hci1 (even though only one of these is really valid), but only an hci0 is seen by rfkill after refreshing to this kernel.