Link. Needs more work but I like this theory. I suspect it’s not COVID specific.

“Campbell added that it was possible people with a tighter blood-brain barrier might be better protected from brain fog should they develop long Covid, explaining why the symptom did not arise in all patients.”

In other words a less tight BBB may be a preexisting condition (genetic?) that makes post-viral neurological dysfunction more likely. So not due to COVID but a variation that makes brain more susceptible to COVID (or other viral) injury.