It's even weirder with Pokémon Trainer!
Since the last time I posted, I have been encountering this glitch more frequently. In some cases the winner ends up being a different player, but it gets really weird when a Pokémon Trainer is in the mix:
I was in Basic Brawl but had to leave before the match started. (I'd never go idle in a serious Team Battle, though, just in casual Basic Brawls.) Another player was also idle, so that left it into a 1 vs. 1. Amazingly neither of us idle players were killed, and the two active players killed each other once to allow Sudden Death. I just barely returned in time to pull off Snake's codec in the final seconds of the normal round. Then when Sudden Death started, I remember the Falco killing the Ivysaur pretty early. However (possibly due to lag) in the replay the Ivysaur manages to successfully evade Falco's killing up-smash by transforming. Falco then kills me (which he didn't do originally since I was being mostly a pacifist calling Snake's codecs.) Meanwhile, the Charizard that shouldn't even be there transforms again, but this time just his trainer's Pokéball remains on the screen for at least 15 seconds! Squirtle didn't even appear until Falco died from a Bob-omb. At this point the speed increases as it often does in glitched replays, but the idle Kirby wins instead.
Moral of the story? I only saved the replay to see what happened later while I was away. You never know what you might miss!