The game seems to be coded in a way where when the patron dies, it shows the "Victory" or "Defeat" ui part.
The thing to note here is that it doesn't stop happening when one of the 2 patrons has died.
Just like deaths, you can keep playing for a few seconds, and the UI will update events, however, they don't count after the first team has a "Victory".
You seem to have killed it after, and very close to the enemy killing your patron.
The "Victory" / "Defeat" splash screen can only be displayed once, so when your team lost, it displayed for a split second the "Defeat" and you killed the patron and it then showed the "Victory", but the game was already over.
I hope this explains it well for you. Sorry for your lost, nice try!