- Annoyance
This happened last time I upgraded, to 6.7.1.
And it happened exactly the same way this time.
I stopped the array, made a backup of /boot, ran update assistant, upgraded the OS, and it kernel panics on boot.
From what I can tell, the update is breaking EFI. But I'm sure that comments here will tell me that its normal and not fix it.
Going back to 6.6.7.