Bug with more than one WD in group?

While farming with a support barb I generally do 12T+ with the click method of spirit barrage but when another spirit barrage WD joined the group my damage seemed inconsistent and single target elites especially taking longer to kill. Visually i was only doing millions/billions instead of the normal trillions unless the other WD was too far behind/dead then it was back to normal.

Just to note the group size is usually 3 with 1 being afk (friend likes to reroll…) so it isn’t the HP scaling making it feel slower.

Has anyone else had this experience and know if there is a solution or at least an explanation.