While battling with a friend some time ago I noticed that Red Card still activates when its helder get killed by the attack that killed him, even though it shouldn't.
To recreate, give yourself 2 to 3 high level pokemons and fight an npc which you've given a lot of red cards to. Make sure you K.O. its pokemons in one attack and battle it. You'll see your pokemons get swapped around although they shouldn't.
Here is a battle I did on showdown with myself to show how killing the Pokemon in one attack doesn't activate the Red Card.
I used the "Frisk" ability to show that the Amoonguss at the start of the battle is indeed holding a red card.https://replay.pokemonshowdown.com/gen9ou-1887083973-05bil6iyqgiy93ui44dfrgncxkju0zxpw