fire resistance dropping in battle

[Strages Sanctus]

Vampire Queen Pretender.
Cast Elemental Fortitude:

Gains 50% magic resistance right?

What could cause it to drop to 25%?

I have to recheck the battle but I think the next spells were astral shield and fire shield...

[Taqwus]

It might not be the only suspect, but Barkskin and its larger-scale Versions, Protection and Mass Protection, will give a 25% fire vulnerability or produce a similar reduction in existing fire resistance.

[Strages Sanctus]

Ah thanks...

I wish we had the option to modify commanders to
USE and DO NOT USE spell lits.