in Luka's defense, at the time when he ran away with a brain-wiped Marinette, she was catatonic, and was under the impression Chat Noir had killed her - it was only when he was recalling the events to Jagged whilst they tried to patch up his face that both rockstars realized the girl had a pulse.
Considering Marinette would be slandered into public enemy number one via Lila/Chrysalis, it really was a good call to keep her from her parents and fake her death with their help. Luka as the new Ladybug cleaned up the mess from there, and as far as her close friends/witnesses saw, it looked like Chat Noir killed her during a panic attack. Since that idea would destroy what little foundation of faith Paris still had left in the heroes, it was basically decided amongst the miraculous holders to keep what happened a secret.
When the new "Ladybug" appeared and told the public that the previous Ladybug died defending the city from the butterfly miraculous, Lila's careful castle of cards tumbled down. Sure, maybe Ladybug did lie about who Hawkmoth was, but what does that matter when she's dead now? Somebody died. A real hero, not someone like Gabriel, the girl that held the city on her back was gone forever, and the slow boil fury Lila was cooking turned to ice in an instant.
All the other heroes could do was stay silent, in spite of what they witnessed, in spite of their suspicions.
Suddenly, Marinette lying to the public seemed very understandable. How heavy it is, to lie to the public for their own good.
Too late now! Oh well! the truth always prevails, right kids?