Unexplained true damage to minions

As shown with my match history, in this custom game. I did around 15k true damage to minions (and monsters) as Ashe, yet nothing in my runes, Ashe as a champion, or elixir (red) nor did I take any dragons nor fight rift herald, to explain from where that true damage came. So as far as I can tell, there is some bug affecting Ashe and possible the Renekton bot as well that's causing true damage to minions/monsters? (Maybe something bugged with IE? Old code leftover affecting minions still?) Match history link: https://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2992412433/35110458?tab=stats (because apparently including it under link=error 500?)