DE implemented a change to make Hand Mortars shots explode on impact, to make them look identical to other mortars.
However, these other mortars have a massive attack in comparison, and are expensive enough that only a few of them can be present in any fight.
Hand mortars have much smaller damage against both units and buildings, and are intended to be massed.
Seeing multiple large explosions amount to little more than tickle damage each on most targets creates a case of visual disparity that the TAD devs had already found the solution to: the solid shot appearance of their impacts. This reduces the amount of visual clutter and gives a more proportional representation of the amount of damage being dealt per shot.
I suggest this to be reverted to how they were back in Legacy, for the sake of visual clarity.
On a side note, I also noticed that dynamite attacks in DE (used by units such as Railroad Workers and Miners in campaign) seem to explode differently depending on the targets: small grenade-like explosions when used on units, and large explosions when used on buildings.
Seeing how these projectiles look identical to eachother and often have identical damage values, I think these explosions should be the large explosions they used to have in Legacy, as well.