#just... what?

Um... what? Like... what? I legitimately don't know what this ask is even referring to.

Like, "doing things behind her back"... uh... are you referring to them changing the birthday party in Crocoduel from being at a movie theater to being on the Couffaine houseboat, in the hopes that it'd get Marinette to finally talk with Luka? Or... I dunno, Rocketear with Alya telling Nino that she's still a superhero against Marinette's wishes? Those are the only things I can think of this season that might even remotely qualify.

As for lying to her, the only ones who have lied to her are Chat Noir in terms of him pretending to be fine when he's not (which hey, while it's good for communication for him to tell her his feelings, he has the right to not do that), and Luka, who lied to her in Wishmaker about not knowing hers or Chat's secret identities. I get the impression that you're specifically talking about Marinette's Girl Squad here though.

As for Marinette never going behind her friends' backs or lying to them, um... yeah? She has? A lot?

She's required to, for one thing. She constantly has to make up excuses to ditch people in order to go out and be Ladybug. She had a breakdown about it in Gang Of Secrets and pushed all her friends away because of it.

Marinette: As long as I'm Ladybug, I can't have loved ones in my life.

Tikki: What makes you say that Marinette?

Marinette: You know why. I had to break up with Luka because I couldn't tell him the truth, it'd be the same thing with Adrien. I can't share my secret with either of them. I can't be honest with my closest friends or my parents about anything. I'm doomed to be a big liar to everyone, forever. What do I do Tikki?

Even outside of that, Marinette will lie to or go behind her friends backs if she feels like it's warranted. Like with Crocoduel, when she tried to get around Alya pushing her to reconcile with Luka by sneaking over to Juleka and begging her to find away to keep him away from their shared birthday party.

If we're going with deception and lying outside of her civilian girl group, there's also all the business with hiding things from Chat Noir and lying to him this season, but that's been gone over quite a bit already.

Marinette hates liars and doesn't like people going behind her back, but it doesn't happen to her all that often, and she is not immune in engaging in it herself.

example: this fuckery