Why do IR missiles without IRCCM ignore flares and reacquire a lost target?

because this game is bullshit thunder. I came from a game with yak41, extending from a mirage with burner off and flaring, an f20 tigershark launched an aim9l from 5km away and it did a 90 degree turn, while i was flaring burner off (and engine covered inside the tail), it still killed me.