magic % bonus on off-hand items (umbrella, grenades) applies inconsistently

on instant cast damage spells (immolate, fionn's wrath), the damage numbers are the same whether the spell is cast with an offhand item equipped or not. happens regardless of immolate spell charge level or the distance travelled by the spell projectile.

on damage-over-time spells (swarm, quake) the %magic bonus is applied inconsistently. it seems to depend per-tic; bees could apply their bonus with the offhand equipped if the target was far away and the bees had a second or two to travel. Quake initially had the same behavior as immolate and fionn's (damage numbers the same regardless of whether i had my offhand equipped or not), but if I put my offhand on while quake was still firing, subsequent hits of quake would gain increased numbers.

since the character sheet shows a flat magic power while items are held, it should be clarified to the player whether bonuses from the offhand are intended to be added to the spell's actual damage or not.