I'm sure your friend would say, it was not a waste of time, just your good deed for the day.I backed up the P5 with Google1 and wiped it and re-did everything which was a major PITA.
Root does work again so I am sure the owner will be happy.
I just got a notification just now a new beta version of Magisk just dropped about an hour after I finished.
Maybe that will make some sort of change, it does show A12 is now better supported in the release notes.
If this fixes the problem, well good for everyone but danggit I just wasted 3 hours of my time.