I don't think it's an issue, as long as they do some other physical activities or sports.
My ds has always hated football, so primary lunchtimes he spent reading on his own. Not great for making friends but now he's older, that's not a problem.
I did have a bizarre conversation with the pastoral care team at his senior school, when they said "we are worried about your ds. He reads every lunchtime."
Me: OK, is he disruptive or unhappy. Is the reading matter inappropriate? Is he rude or aggressive? Is he falling behind?
Them: No, he's just reading.
Me: And the problem with that is.....?
Them: Uhmn....
😂😂😂