Eric (Final Battle) - Bug or Intention?

[Asmodean63]

I play as a soothsay (mage class). After doing the last 3 battles with my allies I tried to battle Eric. When I wanted to cast a phoenix...I realized all my spells (except scrolls) are gone from my spellbook.

I thought: damn, a bug...and loaded an older save game. So I fought again the 3 battles (my spells were there) and tried to take on Eric once more...only to find my spellbook empty again.

So my question is, is this a bug or intentional? Both things would suck, though