It generally takes more than one game for a punishment to be given. This means that if two players are both being toxic but only one of them gets punished after the game, then it's likely because that player behaves like that more often than the other player. This doesn't mean that the other player's behavior was acceptable, just that they haven't behaved like that enough (or been reported in enough games) to be noticed by the system yet.
You can't just ignore one person either or mute them because they jump to general chat and start making claims and making people report you so you have to defend yourself.
False reports will not get you punished, as long as you don't do anything wrong, so there's really no need to defend yourself in chat in this situation. If you feel the need to defend yourself, that's fine, but you need to remember that there's a fine line between defending yourself and retaliation. Responding back with your own negativity and harassment is not defending yourself, it's retaliation, and that is not acceptable as it only makes the situation worse for the other players as well as making their reports against you now valid.
Alternatively, and this is the option I and many others suggest doing, you can mute any players that are being toxic towards you and report them after the game, since now you know that there's really no need to defend yourself in chat if you know you have done nothing wrong.