Did Fane have any points into Warfare? It's a long shot, but maybe that could be an unexpected interaction with changes to letting healing spells do more damage to undead through Warfare.
That said, I'm really having a hard time following your thought process:
So he drank one potion, his Health went down to 35.
Ok, I thought, it's safe to drink another one.
What part of "drinking this potion deals damage to me" makes you think "I should drink a a second one"? What purpose did that serve?
