Problem has been resolved, though I’m unsure why it had happened.
Immutable flag was set on /mnt2/tempraid, meaning that the contents of /mnt2/tempraid was unmodifiable (though decendants could be altered).
I know I didn’t set the flag, and according to BTRFS IRC, scrubs do not set the flag in the event of error.
Problem was resolved with
chattr -i /mnt2/tempraid
Should have thought of it before - in fact, I could swear I checked it, but apparently I did not.