I mean... The kids just go "Oh, it's thanks to Candy our brainwashing was undone!"
But like... I get the feeling, and I could be wrong here, but yeah, Candy just did fall for the brainwash, but everyone else WAS Kate. They just don't know all that's going backstage and assume it was Candy doing whatever it is she did. But if Kate didn't do the whole thing, Candy would've tried whatever it is she did, her still brainwashed friends would've turned her in for betraying the house, and she would've been eliminated- OR forcefed the coffee until compliance.
Point is. "Gosh darn these damn new kids, so ungrateful and unaware what the old folks do for them!"
Exactly, the House system isn't perfect yet as they've observed before, and there is a hole in terms of the Adults having gotten overconfident in the super power of brainwashing. It's one of those cases where having something 99% perfect can be something of a trap, because it's so good it lets people get lazy and fail to consider other alternatives, which then leaves holes a layered approach would not.
However, they aren't
entirely blind, or at least they weren't originally when it was all setup, to the possibility of some varying effects, and the system does have
some layers to reinforcement. Normally there is the initial brainwashing, but then there is constant reinforcement on top, not just in terms of coffee but in terms of rituals, leadership, a snooping culture, and raw physical security. Indeed, we've already seen what Candy
would have been!: Maryrose (granted, in reverse). She'd know the truth, but be completely isolated and alone, unable to confide in anyone else or gain any cooperation. Attempts to escape solo would likely prove futile and end up with the same traps. She'd be worse off in some respects because there'd be no instant knowledge of soot powers under the previous system, and since she's purposefully against her Master odds are decent she wouldn't figure it out or at least be long delayed. So she'd find herself trapped and alone amongst the crowds with a high chance of eventually turning bitter/nihilistic or even giving in and just trying to take the coffee, or else exposing herself and getting disposed of.
So yeah she's got her own thing going now but she doesn't realize she's also a symptom of the system breaking down. They're not getting the coffee and rituals. They're not all spies anymore. The children's leadership is now actively (though subtly) subversive. The entire story up until now has laid groundwork for this, but naturally she's completely blind to it. She probably doesn't fully appreciate the true scope of the danger either, though that could be both a weakness and a strength if boldness means she does something useful.
However at the same time it should be remembered: the Adults still have an absolute, overwhelming advantage in raw power, tactical advantage, and resources. Kate and co are gathering strength by taking advantage of the blind spots, but while glimmers of hope are appearing, they don't actually have all the pieces together yet at all to make a play to challenge things. And the adults are not stupid so much as arrogant and tied up in their own power games, but at least some (Edward) are starting to keep a closer and closer eye on the Children. Candy represents a real risk as well, if it's discovered all the new entrants are free of brainwashing that could blow the entire thing out of the water. I'm assuming the story won't necessarily go that way but things have been overall bright enough for awhile it's easy to forget that this is still a pretty grim and precarious situation for the Children. If Edward suddenly realizes the brainwashing is gone and tells the third floor/GGF and the Adults then immediately moved to suppress it (up to and including eliminating all the Children and starting fresh) it's not clear anyone would make it. Remember too simple stuff like food, the Children's house is totally isolated and completely dependent on external support. Even if they somehow blew up the bridge and could fight off direct attack (nevermind any adults with powers that would let them cross a ravine), they'd starve.