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

Did you faint all four, or just the two that were on the field?

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So, since I do not know for sure I will just put this as a comment.

Perish song affects the active pokemon on from all trainers in the battle.  The only way to avoid it is by switching out.  Since you were on your last mon, perish song cannot be avoided by you switching.  In your case though you had swept all of his pokemon while the perish song counter was going down.  My guess as to why you take the loss in this scenario is because it is possible that Perish song counter goes down after your turn is done.

So, as soon as you confirm your attack the counter goes down, because your turn is over as soon as you choose your move, taking priority over anything that happens after the attack itself.  Thus making you lose even though you had taken out his final mon.  The order of which events occur may happen due to priority of the moves/abilities.  I am not sure if this is how it works, but I will see what I can find.

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What move was used to knock out your opponent's last Pokemon?

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I think it's likely that perish song works like every other status move, except it will always count down at the end of the turn, no matter what. I don't think any other status move, weather, or condition does this

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Are you sure you actually beat all of the opponents remaining Pokemon? When your opponents last Pokemon faints the battle will end and any end-of-turn things like Sandstorm or Perish Song don't take effect.

https://replay.pokemonshowdown.com/gen8doublesou-1378059664
In this replay, I try to recreate your situation. When Amoongus makes the last Pokemon faint, the Amoongus user wins and Perish Song doesn't make Amoongus faint.

https://replay.pokemonshowdown.com/gen8doublesou-1378059664
In this replay, Amoongus doesn't make all of the opponents Pokemon faint and due to that Perish Song activates, making Amoongus faint.

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The perish song counter counts down after every Pokemon has got a chance to execute it's move. (But couldn't, suppose was paralysed etc.)  KOing the final enemy gives recoil damage too. And fainting due to that can also cause losing. So,the perish song counter keeps on counting till the end of the turn except if the victim is withdrawn, because even though the enemy has fainted,the battle effects are still lingering.

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This answer doesn't say anything OP doesn't already know -- they want a precise explanation of the Perish Song mechanics that caused them to lose even though their Pokemon was the last remaining. I edited the title to make this clearer.

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The question says all enemies already fainted before perish song counted down to 0. I don't think anything is supposed to linger after all enemies faint.

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Perish Song must have lingered after all the enemies fainted, or else OP wouldn't have lost. But they already know that.

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Does it have anything to do with how much HP your pokemon has compared to your opponents last pokemon's HP JUST BEFORE you KO your opponent?

I mean, you did get the ko, but after that, the perish song perished your pokemon, and you said you got the lost.

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Was your opponent’s last Pokémon KO’d from Pollen Puff or would it have died from poison or some other field or status condition?

What was your opponent’s last Pokémon?

Was both your Amoongus and your opponent’s last Pokémon under the effect of Perish Song? :P

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Well that thing happened to me too. Though I wasn't the one who used the P.Song. Maybe it still lingers on the enemy side?

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I don't think the user is on this website anymore, they asked one question and did nothing else

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@cole and @Master By any chance do you have videos or replays of this thing happening? It's very likely that you just remembered wrong.

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I'm p sure the slower mon wins as they go faint last.

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Did you read the question? It says the asker lost the battle even though his/her/their/its Pokemon fainted last.

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No I didn't oops mb.

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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.