Ahh yes I've seen this image too. Must've noticed it but it did escape my memory. Makes this a curious thing then but I feel the explanation is small and trivial. Maybe they're just covered by a shell to protect it since it's not like they need to use the sensors at an airshow. That's if they decide to keep it covered for the actual performance.
Could be as simple as that or could be the whole thing is a ruse and there is no real "J-20". All a military strategy to get the enemy believing in virtual strength.