I have a brain injury that causes visual agnosia, so for the most part, people's faces are just a big "blob" to me when they talk. It's like I can see their face - I see two eyes, a nose, lips, etc. but the part of my brain that interprets the expression scrambles it up and I don't get actual "data" from it. I don't actually remember ever really putting that much stock into people's facial expressions, so I am not sure how much of that is completely due to TBI, or due to other issues (such as PTSD or RAD).
It's been somewhat of a pet theory of mine that trauma, especially during infancy/early childhood, causes social impairment and such impairments can be so severe they can even be misdiagnosed as autism. I've known quite a few people who were severely isolated as children and thus demonstrate the vast chasm between "asocial" and "antisocial." (I have traits of both - conduct issues and a lack of proper socialization due to severe solitary confinement from 0-6).
This is something very rarely studied nor understood as the conditions required to invoke these responses would be unethical to reproduce experimentally; but there are cases like Genie, Victor of Aveyron, etc, that discuss so-called "feral children." The idea being if a child is not properly socialized (or who endure such extremes of abuse) they will quite simply fail to become civilized, and are in essence "feral."
(To a lesser extent, we can see this in kids that have RAD - there is antisocial behavior, but there is also asocial behavior, and they are different.)
All of these things would have drastic impacts on how you perceive facial expressions and respond to them, regardless of organic physical trauma.
(You may also be interested in something called prosopagnosia and aphantasia, which are more along the "normal" spectrum of human experiences - typically not caused by organic injury, they refer to the developmental/congenital inability to remember/identify individual faces, often including one's own face, or create images in one's mind, and are further linked to deficits in sequential reasoning and manipulation of variables such as in solving a math equation "in your head.")
Essentially, this is a broad issue that could be caused by a variety of things, but certainly can be relative to PTSD. After all, we are supposed to provide children with the foundations for expressing empathy and emotions, but if your primary caregiver is abusive, you will not internalize the same information about those states as your peers.