Tried using a Heavy’s rocket, and in the past, grenades, in multiple situations where the blast radius shows a kill or serious injury to the enemy. The actual blast does little to no damage. The enemy is within the blast radius, but usually has some small destructible obstacle between the blast origin and the enemy.
Last time I used a rocket, it showed a kill, but the enemy received no damage. Sorry, but due to circumstances, no pictures. It’s been prevalent enough that the specific instance doesn’t seem to matter, with the exception of there being destructible objects or terrain between the enemy and blast origin.
This happens quite often if, as you say, there is cover between the explosion and the target. It also happens if the aliens shield is blocking the path. Both I think make enough sense.
I started to read the damage to the alien, as the “best case senario” for your shot. Anything blocking the force of the explosion will remove some or all of that expected damage.
I have started making sure I land my firecat shots behind crabmen because the shield makes the damage unpredictable. When you land the shots behind them damage is almost always maximum.