I'm not entirely certain how it works, but twice now as Rune, I got insta-killed after my teleport completed if I was hit by a knockback ability just as the teleport goes off.
In neither cases was my shadow anywhere near a killzone, and I was not staggered. I did not visually see myself get knocked back after the teleport completed, just instant death.
Sorta sounds like it could be similar to that Zentaro bug where he dies during his ult.
(I am playing on PC.)