There’s no conclusive sum of factors though a big one is the Amygdala overactivation (and, if relevant, it’s growth in comparison to other parts during the onset of symptoms) causing difficulties with encoding, storing, and causing behaviors that worsen recall (and whatever affects recall is usually actively affecting the other two components of memory as well). The Amygdala in this situation, in comparison to the sick brain, is what is causing the memory defecit- as opposed to a random assortment of possibly damaged neurological components, that could include the amygdala (which is to say being sick can make neurons, parts of the frontal lobe, the back area of the brain responsible for vision, or all of them become damaged in some way, for example) and while the effects may be the same, PTSD may have been a risk factor for or an effect of receiving the brain injuries associated with a virus more times than not

Just to further elaborate, PTSD is a mental illness (though it’s common coupling with brain injuries is something if interest), whilst memory degradation from viruses, specifically from COVID, seems to be from either inappropriate immune system activation or neurological (like described above)/systemic damage (think possible gut biome or other body damage that may make it harder for you to do anything that was easier pre-COVID, like joint aches for example)

(for those reading whom are struggling with PTSD, please remember that it is a normal reaction to abnormal circumstances)