I would say that "malgendering" as a phenomenon describes something that happened specifically to people who's gender/sex is already under scrutiny or seen as illegitimate.
It's not just about gendered stereotypes, it's about them being wielded against someone who the wielder knows/believes the victim's gender is dubious in some way. It is a way of punishing them for identifying with a certain gender by gendering them correctly but punitively. The correct gendering becomes a punishment that harms the victim, which can have the affect of making them feel unsafe in their preferred gender presentation, making the act of living as their gender identity traumatic in itself.
As a historical example, this fictional story from the 1850s depicts a young masculine child seen as a girl, who is (as a punishment) made to wear boy's clothes for a week & subjected to cruelty from their brothers. Their brothers say that "if you act like a boy, we'll treat you like one," but in practice this means physical and verbal abuse. This punishment goes on until the child breaks down under the scrutiny and dedicates themself to repressing their masculinity, because they have now come to associate their transmasculinity with being humiliated and hurt and vulnerable.
So if I had to sum that up in some characteristics of malgendering that make it distinct from harmful gender stereotypes in general:
the victim is someone who's gender/sex is perceived as dubious, suspect, or illegitimate
the perpetrator percieves that this makes the victim uniquely vulnerable to gendered stereotypes, because they do not have access to "proper" manhood or womanhood.
as a result, the perpetrator's correct gendering of them allows them to assert power over the victim by saying their gender will be validated only when their gender is portrayed as negative.
while proper gendering helps empower the gender-marginalized person by affirming that they can exist socially as their gender and be part of society, malgendering disempowers the gender-marginalized person by only allowing them to exist as a cautionary tale for their gender, or as the butt of a joke about their gender. their gendered existence in society is not in their control, and it is wielded for the benefit of others
as a social phenomenon, the ultimate goal / end point of malgendering is either detransition or suicide. while misgendering tells the person that they will never be allowed to exist in society as their gender, malgendering tells the person that existing in society as their gender will only cause them pain, isolation, and loss of autonomy; either way, the conclusion many victims come to is that they should either live as society demands they do based on their perceived sex/gender, or not live at all.
Confirmation bias can play a role, but that's not exactly what is intended to be highlighted in describing certain actions as malgendering.
