Cool. Cool cool cool.
Looks like the issue I was experiencing was resolved by
https://github.com/ceph/ceph/pull/34745. Didn't know encrypted OSD's weren't
supported at all. v15.2.0 did seem to handle them fine, looks like 15.2.1 and 15.2.2 have
some regression there. Under 15.2.1 OSD's are now failing to come up on reboot without
resorting to killing cryptsetup.
Thanks for the link, I was looking in the completely wrong place!