https://humankind.fandom.com/wiki/Mortar

"Heavy Weapon

Ranged-combat Unit, must have line of sight to attack. Can only move or attack in a Turn turn. Cannot retaliate when attacked by a close-combat Unit. With Entrenching Tool, gains "Dug-In" status if they do not move during their Battle round."

Mortar does retaliate against melee, or cavalery (which a melee unit). Contrary to the description.

It contribuates to make this unit and overpowered one.

Hello !

While nobody directly answered, we had seen your bug report and catalogued this as a bug !

The fix has now been done. It should be available in one of the coming updates :)

Thanks a lot for your help :)