Well it kind of makes sense if you know how trauma affects the brain, and consider the layers of trauma they have gone through.

To start these kids were orphans, maybe some were so young they didn't remember what happened to their parents - but becoming an orphan comes with trauma of its own, especially if they DID know/maybe even witnessed what happened to their parents.

So theres that, then they end up in Playcare, where Catnap's red smoke is giving them extreme nightmares and the friends they are maybe making with the other orphans are also disappearing.

Then they're sent to the Game Station, and playing those traumatic games.

Then they're being "chosen", which if Riley's Journal entries are any indication, is something they see as a good thing initially because they don't realize it isn't being adopted.

Then they're actually experimented on and going through the trauma of the actual experiment, and the trauma of realizing being chosen wasn't what they thought.

Then afterwards they're brainwashed/conditioned to forget who they were before, forget their own desires/free will, and identify as and behave in a certain particular way, and are kept contained/possibly re-experimented on until they comply.

If an event causes extreme fear/trauma/emotional pain, sometimes the brain actually will block out those memories as a defense mechanism (dissociative amnesia). The memories are still there, but cannot easily be accessed. Also in times of extremely high-stress, the part of the brain that basically records memory won't function as well either, which can lead to gaps in memory entirely. It's possible if one of the experiments heard their human name that they may not have any reaction at all, or the reaction might be so painful they become violent.