I have found that during group/coop play when you gain enough xp from a mission to level up a char you do not get the 2nd Level up screen upon mission completion. Any coop mission produces this same result if char level up is achieved.
Only the initial mission loot screen shows. You do however still gain skill points and attribute points but lose out on all loot provided for the actual char level up.
This is not an issue during solo play when leveling a char and you do get both the mission reward and char level up screen (if applicable) as far as i can tell this is affecting all classes.