As far as how the game should work, it should be a tie. But like you said, due to how coding works, the game doesn't recognize a winner.
Also, looking at the screenshots posted by GG, it looks like she should have won. Kills had 2 health left, and she ended her turn. The poison put him at 0, then the creatures attacked, putting him at 21. If the poison and first dragon's attacks coincided, they would have canceled each other out, and he'd have 12 health due to the other antimattered dragon. The same would be true if the poison didn't take affect first.
So, it appears that this really is a bug, since his health actually did go to 0. I'm assuming the game doesn't check for a victory until the first creature slot has made an action?