Hi, I’m wearing Mythical Dread-Mask of Gurgoth.
So, mortar damage is shown as fully lightning in the Mortar Trap tooltip.
When I equip additionally Mythical Mark of Calamitous Desires, Physical Damage of the mortars should be split to 50% fire and 50% lightning. That is exactly the damage applied according to grim internals (see pic on the left, only mortar trap attacking dummy in background). But the Mortar Trap tooltip shows (50%) physical damage, which is in fact fire damage.