I found two issues, but I think, under the hood, they might be the same issue, but I'll leave that up to the devs to decide.

Firstly, the scripting command knowsAbout remains 1.5, even though the whole group is dead, which is not expected behavior, dead people / groups do not know about anything IRL and so I'd expect them to also not know about anything in game.

Secondly the trigger settings as attached (any player, detected by OPFOR, with a Time Out of 20 seconds), will still trigger, even when all enemies died, this leads me to believe the two bugs are related, even though I don't know for certain that they rely on the same command.

I also attached a sample mission (SQF), I shot on the enemies below, missed a couple of shots and then killed them, well within 20 seconds, but the trigger still fired.
In the sample mission, script command: enemygroup1 knowsAbout player was used to determine that the group still knows that the player is there, despite them all being dead.