Mundo's E sometimes does not have bonus damage right away on towers!(GAMEPLAY)

So Dr. Mundo's E is an auto attack reset and I use it to deal more damage to towers. The way you do it is have no E effect first, auto attack tower and then press E right after the auto attack for a quick two auto attack on tower. However, let's say the first auto attack deal 75 damage but the second aa also deals 75 damage when it should have done 125. Sometimes the second auto deals 125 as intended but sometimes the second aa fail to do the bonus damage from E and deals 75 just like a normal aa without E active. It really hurts to deal less damage and the extra damage piles up and can mean a lot, please fix!