What happens if you short between the two sides of the spot? Does it attenuate then?

I'm thinking that it's not meatbag attenuation, but detuning of the antenna because it suddenly becomes much longer when your skin provides a conductance point between the two elements.

If you could run a long piece of wire between the two sides, such that there was almost nothing that could possibly interfere with the signal strength at the antenna, and it still causes a problem, then that would reliably disprove the meatbag theory. Maybe try with a capacitor, too.

Maybe the reason that some people are having problems when others aren't is that some people's hands are sweatier than others.