Platform, device version and operating system:
PC, Steam client, Game: 4.2.0.07, OS: Win 7x64 SP1
Screenshot or image:
What you were expecting to happen, and what actually happened:
Units that are submerged cannot be damaged by area weapons or other effects. That’s okay.
However, the damage values should display a proper damage value to submerged units, like a 0, whenever this effect is applied on such units and all the damage is ignored.
The Dawnbringer weapon does 40 damage to all units.
If all of the enemy units are submerged, the weapon will trigger, but its effects will be ignored, 0 damage values should be shown on all 4 units, and not 40 since no damage was done to any of them.
This alone creates an illusion that the enemy took the damage. Just watch a recording of my video.
How often does this happen? When did it begin happening?
Always. Probably since the beginning.
Steps to make it happen again
Best course of action would be to use all four Leviathans on the opposite team.
You can use the setup that I used in my video. The choice is yours.