So, one of the drives dropped off the RAID5 array some time ago. Got the drive back online after a reboot after I noticed it.

However, the drive is perfectly fine. 🤨 So after figuring how to add the drive again, it was added back in the array.

dmesg: md kicking non-fresh /dev/xyz from array
run - mdadm /dev/md0 -a /dev/xyz

Took a bit to resync, but the array is fine again.