Minor bug, the clairvoyance line is not adding any mana when you select it.  

The description of the spell is also a bit ambiguous, as it describes both a per-battle perk of bonus spell casting and a mana bonus which I assume is meant to be a one-time perk (it'd be too OP otherwise).  It's a perfectly fine perk if you just chopped off any mention of mana.

Edit: Both the male and female spell casters have skill-line called clairvoyance, but priestess line does not mention mana bonuses.