Taking a shot in the dark but I believe it's because when you switch to a different weapon like a wand, it uses the mage formula / wand multiplier or something.
If a warrior equips a mage weapon, they use a different formula that always makes them do 0 rounded up to 1 since you can't do 0 damage to an enemy.
If a warrior equips a mage weapon, they use a different formula that always makes them do 0 rounded up to 1 since you can't do 0 damage to an enemy.