This, I believe, is tied in with your HP bug. It seems the perks are told we get from fusion aren't actually all recieved. So we don't gain 5-10 HP we are meant to recieve from fusion and we don't recieve 1-2 damage we recieve from fusion.

This is more noticable with the HP. At the beggining of some games, after you've made your first move, you can see your HP might be lower than your max HP. Something that needs to be fixed.

yeah, also the same with some weapons, for example, on my 30 lvl legs, they do 84 dmg, teoretically, and in fact they do 82 dmg

Currently, there is a bug with the fusion core which makes the game consider all the items has 2 less power level than they actually have.
It happens in multiplayer matches and this goes for every item that can be upgraded with the fusion core.