Thanks @john, if you put it in an answer I'll accept it.

What I'm seeing doesn't seem to match the widely reported bitlocker on 22H2 issues. But it seems something may have updated my Intel driver's unexpectedly past support for my chipset. This might explain it, but doesn't explain how it can still see the raid at all.

@Ramhound, machine finally came back up, still exhibits behaviour, updated answer with Win10 version.

@PeterMortensen, thank you for the edit. This one is not useful for current without modification.

I finally built mdadm-3.2 and tested it on Debian 10 with kernel 4.19: "mdadm --detail --scan" returns an empty string for me too, even though "mdadm --examine some-disks" still shows my isw_raid_members correctly, so perhaps this is due to a kernel or udev difference instead of mdadm?