Garen ult and true damage bug

I have had a friend play a few games of Garen in the last few weeks, and we have started to notice something strange with him. His ult seemed to sometimes not deal any damage to the enemy. I first noticed it when it was used at the same time as Zac ult, and thought it may be an interaction issue. However, in a recent game, it sometime seemed that Garen ult did not do any damage when it was used. Another thing I had noticed is that Garen only dealt around 500 true damage in that game, which seemed rather low with his villain passive. I haven't been able to find much discussion on the topic, so I figured I would post here and see if anyone else has had this issue.