>>529176590
The truth is we just don't know. While I do think Arbiters are limited, I don't think losing one is a massive defeat which pushed the head back years. You can also explain it as Ayin and Ben got away with it by vivisecting Garion's brain to find out how to dodge the Beholder's tracking of their taboo violations, so as far as the Head was concerned, they were no longer breaking taboos after that.
Interestingly, Zena and Baral didn't crash through the roof of the library UNTIL Angela abandons her quest to become human, so it seems like they may give you some leniency if you had no say in your taboo violation, as Angela didn't ask to become human, but were actively working to be compliant.
but I don't think there are any real substance to these mysteries yet. The Head is still just this ominous and obscure power that acts on strange rules.