[Gameplay] Lulu gets invulnerable

I had a game where I was playing Garen against Lulu then her HP was low so she casted her ulti on herself then i gave some few damage before i casted my ulti. Then when i casted my ulti on her I thought she was gonna die but what happened was she got invulnerable. Im not really sure if it's a bug but i've never seen lulu getting invulnerable. It only happened once but maybe it will happen again. we never know.

P.S. Her teammates were Blitz, Jinx, Trundle and Talon and we know they have no skill like Kayle's ulti so im just wondering why that happened.