I just played a match with someone from the US (I'm from EU) on a mission with -30% damage dealt modifier and my damage was severely lower than it should be. I play impossible tarots and 1 shot most elites with my sniper but in a hard difficulty mission in co-op I needed 5 or 6 shots, it would display 20K criticals and the enemies would have 5K/6K total HP but they wouldn't die. What's up with that?
Edit: Just to add up, it isn't just the sniper shot that has the damage reduced, it's all damage dealt.
Edit2: Tested this with another EU player and the same issue happened.
Edit3: Turns out sometimes I can snipe an enemy that I only deal 2K damage and has double or triple the amount of health and dies where other enemies with far less health don't die when I should severely overkill them.
Edit4: It seems that all ranged weapons are bugged.
Edit5: After some psyker testing it seems that the same issue afflicts the character.
Edit6: The damage caused by all psyker spells seem to suffer the same issue as every other ranged weapon but their warp rods do not.
Edit7: There may be some issues with critical damage, some non crit damage that would finish off an enemy actually works and kills that enemy.