How is the winner of the battle decided when Perish Song is on its final turn?

Question

My opponent used Perish song when I had two mons left to his four. I swept all of them on the last turn and immediately after fainting his last mon, the counter hit zero and my last mon (only one remaining) fainted from perish song. I got the loss. This is frustrating to think I beat it and lose anyways, so if someone could explain it better it would be much appreciated.

Edit: Perish song was used when I had two left and he had four. I swept all four and finished the fourth on the last turn of Perish Song (counter reached 0 same turn). I believe Amoongus was my last mon and Pollen Puff was the finishing move used in trick room.

Editor edit:

Only one Pokemon fainted from perish song.

Please do not say stuff like "When multiple Pokemon faint from perish song, the fastest one faints first."

Comments

Isn't this a dupe?
https://pokemondb.net/pokebase/205040/how-does-perish-song-determine-which-pokemon-faints-first

I mean, the BA does kind of answer his/her/their question

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Does this count as not answering a clarifying question?
@Gatik I made the description easier to read. Can you read it now?

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@GeniusGatik No, that question doesn't ask who wins the battle.
@sumwun I think it's feasible to give possibilities for what happened without knowing the answer to Staka's questions.

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I checked with a battle mechanic researcher on Smogon, and both of us agree that the only feasible possibility is that they remembered wrong. Can I answer with that and leave the answer here until they give more evidence that it really happened?

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Yes, that's a reasonable thing to do.

Answer 1

If Amoonguss actually knocked out the opponent's final Pokemon, then no, it's not possible - the game would end, so the end-of-turn effects wouldn't happen, so the Perish counter couldn't decrease. The other person probably didn't actually KO the opponent's final Pokemon. You'd need some cartridge footage to show where the mistake is.

source
I think you most likely just remembered wrong. I'll leave this answer here until someone brings up more evidence showing that it really is possible.