I suspect Vastilok heavy slam blocks itself due to LoS.
You can see this more when you heavy slam twice in the same location, but sometimes even once. Enemies just sometimes aren't hit. Like, when I place the marker on the enemy, I've been 1m next to them in the Simulacrum. I'm guessing it's LoS cause when I slam the very center one, the one behind doesn't get hit, but the ones diagonal do, even though they are further away. It happens more when I'm looking straight down.
This is only for the heavy slam, normal slam will hit everyone normally. This is funny cause the heavy slam is 7m while the normal one is 5m.