Systemd is not able to return to the initrd on shutdown, which prevents it from disassembling mdraid storage containing the root partition. See this issue for reference: https://github.com/systemd/systemd/issues/19812#event-4841586197
It's possible this issue only happens when there is a bcache cache drive on top of the mdraid array. I have a couple other mdraid arrays on my system and I only see this error message for the one containing the bcache cache.