Fountain turret vs Ryze ult

So there's this match where we're losing and Ryze ults himself into our fountain, along with a bunch of minions and then Riven dives too. The fountain turret starts aggroing the minions, but then Riven and Ryze start attacking us as we spawn and the fountain turret never switches aggro to them. It didn't attack them until the last minion was dead so they could easily stay like 10-15 seconds inside the fountain without receiving turret damage. I have the replay but I don't really know how to upload it and post a link. I think this might be a bug so I want to know if something similar happened to someone else.