In my case I've found that I get a brain fog when I'm mentally tired. Physically I can be full of energy and have no problem with practical tasks or working out etc. I've found that social contact, particularly talking, is very mentally draining. This contact could be out socializing in a group or in a work meeting either on teams or in person. I'm generally ok on a 1 to 1 basis like a date, it's when there is a group or I'm moving from 1 person to another in fairly quick succession.
The best way I've found to deal with it is not to be tired. Getting a full good nights sleep and also when I feel tired I don't go out or I call it a night and go home. I can push on and go out or stay out longer but I will pay for it the following day or so. Another thing is to not be hungry while I'm out, sugary drinks can help - beer, although I don't drink much, or soft drinks, just as long as it has sugar then that helps but isn't really a replacement for good quality rest and sleep.
I've not really noticed this before, only since getting covid so I've been trying to understand it for about 3 months.