[Gameplay]- Rammus interaction with Undying Rage?

This is a bug where I was playing as and was able to kill a while his Undying Rage was active. It occurred twice in the same game. The first time, I was ganking top lane, I think my ally was a Gangplank. I had red buff and my ult active and my Defensive Ball Curl active and was auto attacking. Less than one second after Tryndamere ulted (I heard the sound and saw his rage bar increase) he died. Later, it happened again, this time about 3 seconds into his Undying Rage. I'm not sure if it was red buff or Tremors or my autoattacks or the returned damage from defensive ball curl, or maybe even the return damage from Gift of the Toadstool or an interaction with Puncturing Taunt. It may have even been Gangplank's passive, but the second time, I was pretty sure it was one of my autoattacks that killed him.

Steps to recreate:

1. Create a custom game with someone with you one one team as Rammus and the other person on the other team as Tryndamere
2. Get both of you to level 6
3. Have Rammus attempt to kill Tryndamere while Undying Rage is active with various combinations of the above circumstances
4. If none of the above methods work, try adding a Gangplank and try various combinations of his passive and Rammus' abilities/attacks

I probably wouldn't have thought this was legitimate if it hadn't happened twice. It would be great if someone could investigate/confirm this. Here is a link to the match history for that game: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1970063211/43497197?tab=overview Thanks!