Yeah, I was thinking you’re essentially taking a diversity antenna set and shorting the middle of the two antennas together through your hand - that will play all kinds of havoc with the impedance! Icky!
Just a few mils of polycarbonate over the antennas would solve the problem, but then the pretty outside is ruined. Form should follow function, not the other way around.
There are a number of plastic or leather cases available. Would that solve the problem?