... The game still counts the defender as the winner, LOL.

Probably a thread about it has already been made in the past years, but it's the first time that this ever happened me. And totally by chance as well.
Both Venom and Annihilus reached 0% HP almost simultaneously because of DOT: the game still played Annihilus's winning animation, despite him having exactly 0 HP remaining at the end of the fight.
I honestly don't know if there is some tiebreaker criteria/rules, or if the game always prioritize the defender as the winner in these rare occurrences.