After 1.05 patch I noticed Itumaak was dealing insane amounts of damage and I contributed it to the animal companion fixes at first but after it critted for 140 I went to check the character screen, only to find the animal compaion combat bonuses buff stacked multiple times. I do not recall seeing it before the 1.05 patch.
Unfortunately I do not have a save from times before the bug, so I can not really tell what went wrong, but I have noticed that it happens when I Quicksave and then load the said save in Dyrfords Crossing Anywhere. Each time the game ends up giving me an extra stack.