The idea that an identity is only valid if it is stable affects not only queers but also the mentally ill. To have a mental illness and then to lose it is seen as something between betrayal and appropriation. This is one factor driving the resistance of a significant portion of mentally ill people to do any work toward recovery, even if they can clearly see that in their current state, they are miserable and, some of them, hurting other people.
A form of dichotomized thinking - the worldview that splits everyone into the category of abuser or victim - relates as well. To be mentally ill is to be marginalized. To be marginalized is to be a victim. And by their logic, to be a victim is to be incapable of harming others. This could be a factor in why some number of abusers, especially online bullies, will point to their diagnoses when called out on their behavior. It definitely feeds into a resistance toward recovery, as to recover could mean no longer being mentally ill, ergo no longer a victim, ergo an abuser.
To continue being validated as mentally ill in spaces where fakeclaiming is common, people will performatively speak of their behavior and mental illnesses in a way that most closely aligns with existing stereotypes within the community, even if that does not match the reality of their illness or if it actively harms their recovery.
Additionally, many of the users involved in these communities are young, often still dependent on abusive relatives and cut off from older members of the mentally ill community (sometimes deliberately, as ageism is common), meaning that no one is around to set an example, and members attempting to correct them are mocked and accused of inappropriate behavior.