I had the feeling it might be related to terrain elevation, I think I’ve even mentioned it once? Okay, it may make sense with him, but what about grundleplith (triple bite doesn’t connect for entire fight in like 9 out of 10 first-time encounters) or ancient shambler (can’t remember when I saw his punch connect last time)?
And what’s makes it even harder to test is that it just seemingly stops happening at all when revisiting these bosses with higher level char 
Or at least this is how it went last time I tried.
P.S. Zantai said he’s starting to think it may be about potato PC, I can tell it’s not that 
Unless GD doesn’t like overpowered PCs as well as underpowered. Getting much better rig solved evade bug (when evade randomly just doesn’t move you anywhere but still goes on cd) to me, but not this monster melee hitbox one.