Bug or actual gameplay, I would like it to change

Yesterday, during a ranked match, a buddy and I saw something quite weird. The situation is my buddy is jungle as amumu, and Im mid lane as kennen going against an anivia. We're all about Lvl 7-8 and my buddy comes in for a gank. Hits his snare no problem, ults locking down anivia. I then flash and ult to maintain the onslaught. After my ult, Anivia is left with about 5-10% of her health. She then starts to teleport to escape. We both end up causing anivia to initiate her passive, that is dying and then becoming egg form. Im thinking guaranteed kill. However, the teleport finishes and I see the egg just chilling at top turret and then Anivia reappears unharmed. Please Riot, this could apply to Anivia Zac whoever has this kind of passive, if someone starts to teleport to try to avoid dying but ends up "dying" initiating the passive, dont let them still teleport. "Dying" should cancel the effect