Neuroendocrinologist (researcher, not doctor) over here! Personally, I don't think the SGB is a total red herring: I think that you're right that a TBI causing memory problems is the likely proximate cause, but based on a bit of research it looks like SGBs can fuck up your amygdala something cruel which would potentially disrupt the Papez circuit. Given that the Papez circuit is key to processing specifically emotionally charged memories, I don't think it's necessarily impossible that the SGB procedure contributed to this issue.
My hypothesis here is:
TBI and/or the neurochemical and neurobiological shifts caused by PTSD created some form of prosopagnosia.
SGB disrupted Papez, fucking up the processing and maintenance of those memories.
Deployment meant that contact was extremely limited, dwelling on those memories was limited due to time constraints and trying hard to avoid reminiscing to avoid PTSD episodes, and all in all it created a perfect cycle of shit for weakening and keeping weak emotionally charged recollections of faces and names.