its not directional vs explosives, if youre in green cover youll take half damage from grenades no matter where the grenade lands.
I just checked myself in CheatCommands, from what I can see it is
kinda directional versus grenades, but it's a bit weird. Were it simply "In heavy cover, take half damage from grenades", it would be impossible to kill off models at full health with it yet I did manage to kill one Panzerfusilier with an RG-42 - however, it seems to only circumvent the cover if placed
very behind the heavy cover, and so placing the grenade right in the middle of the squad behind heavy cover will still have it count as directional to the heavy cover and thus prevent it from killing models at full health. I got the model kill by placing the grenade a bit further back, I imagine from placing it in a spot that is completely outside of the hitbox of the cover (as can be seen by how far the green dots go from the wall).
Of course, placing the grenade so far back somewhat reduces the effectiveness of it anyway since it is farther from more of the squad, though it did look a bit more effective to do this instead of just placing it in the squad's center to me.