It is a bit sobering that Mr. and Mrs. Johnson believe Janie when she tells them about her quest to confirm if she's Jennie Spring or not. Then again, they had to recount to Janie that her "mother" dropped her off one day explaining she was on the run from the cult that brainwashed her, and they can tell how bad it sounds on a retelling because even if that story were true, it's a woman abandoning their child to be raised by her parents. While Janie thinks that her parents accepted Hannah's story without question, they actually didn't. Their bigger concern was raising Janie and comforting her. It's logical that since Hannah has lied to them before, she lied that Janie was her daughter.
Though Janie assumes that she accepted the Johnsons as her new family without question, that likely was not the case. As a three year old, she must have had moments when she cried and screamed about wanting her mom and dad. The Johnsons assumed that, by "Mom" and "Dad," she meant Hannah and the father that must have given her a daughter. It would have been perfectly ordinary for a child at Janie's age to cry when she was separated from her "mother" for the first time. The Johnsons soothed her with the explanation that her mother and father were unable to take care of her at the time, and that the Johnsons would be her parents. As Janie grew up, she eventually forgot the parents she had for the first three years of her life.
The Springs don't know that "Jennie Spring" is allergic to milk. Yet the Johnsons, who for all intents and purposes unwittingly adopted her, do. While it's likely they would have found out eventually, it shows the Springs can be oblivious to rather important details, and it was how Jennie vanished from under their noses.
Book 2 has a cop refusing to arrest Hannah in front of Janie's biological siblings when they demand it. They say that she's been punished enough and advise the kids to go home. Safe to say if Hannah had been arrested, less gaslighting would have ensued in later books.
Thats not the case. The cop points to a random homeless person and says that she is Hannah but he is actually pointing out that the person might as well be Hannah, because Hannah has no doubt been punished by life in a similar way to the person they point at.