Panic for health bar bug

Greetings,

Yesterday while playing with a friend, i saw a bug that made me shock a little. I was walking towards mid and my frien told me he was fighting with someone (obviously ingame) and my camera was spectating him, but in a moment, instantly his life bear dissapeared amd he was dead, no one was attacking him NO ONE, and i wasnt lagging , he was lagging, but why does that happened, he told me he got away from the fight and he was alive but 10 sec later his health dissapeared. I think its a bug and possibly it has only happened to me but i think it needs to be fixed cause i got scared hehehe.