Strange Morde ult bug (or health bar bug)

Last night I ulted a Gnar in Mega Gnar mode with Mordekaiser, while a teammate using Vlad ulted him and ignited him, and his life actually increased. He had a Hexdrinker, but being ignited, vlad ulted and supposedly losing 26% of his max health(on top of any other random attacks that flew during a short fight) should have easily dealt far more than the shield, but it looked as if the shield wasn't even decreasing. My ult played it's animation and sound, and went on cd, but otherwise it was as if it didn't happen at all.

I've seen many occasions lately where I'm getting the impression that my locally displayed lifebars are not properly in sync with what's actually happening, and this could be another instance of that. This is quite a horrible bug as it has caused me to die countless times when my client tells me a target squishy is at 50 hp, I basic attack him 3 times and he's still supposedly at 50.