The original Red was enough of a greedy asshole that his behaviour is too much in character to warrant further examination, maybe? Or Green Keeper suspects, but also knows that he needs ironclad proof in order to accuse another Dragon Keeper, and doesn’t have it yet? Or he’s saving it until after they’ve dealt with the other boss monsters? Or some other hidden reason, possibly having to do with why the Keepers’ fancy ultra weapons appear to be organic?

Part of the reason D was fingered so quickly was that he really isn’t that good an actor and doesn’t take precautions when he thinks he’s alone or with people who already know. The boss monster may be better at faking humanity.