To me, it’s the lighting. It almost looked green screened to me for a bit, because the sculpted foreground lighting and the flat (natural) background lighting on the stadium were so different. I don’t know if that’s what you’re reacting to, but that’s what it was for me. I mean, these things are usually lit, but the lighting was a bit more dramatic than I’m used to seeing in video.
It also looked weird because during the first song everyone in the stands were holding red and yellow placards I think. It looked, at first glance, like empty red and yellow seats.
My reaction as well - I couldn’t figure out why, but I think you’re right about the lighting. I kept thinking that it seemed like people were edited into the shot.
Related to my question in the OP, why do people in the stands do that? Come halftime, I’m going to the bathroom, and then buy a bratwurst and a beer. I’m not going to sit there and flash placards.