Taking another look at this with more time, I see this on boot:
Aug 25 18:48:25 Moulin-Rouge kernel: BTRFS: device fsid 46c809d9-2d8f-48ad-be38-dff41668218c devid 1 transid 2879 /dev/sda1 scanned by udevd (752)
...
Aug 25 18:48:25 Moulin-Rouge kernel: BTRFS: device fsid 46c809d9-2d8f-48ad-be38-dff41668218c devid 2 transid 814 /dev/sdc1 scanned by udevd (765)
sda was the old cache device, sdc is current parity, note that sdc is device 2 from a pool with sda, trans id differ so it means they were in a pool some time ago, not just before this boot, while it's still a bug looks like the issue happened after a very unusual sequence of events, i.e., parity being in a pool with a former pool device.