The best way to think about thermal cameras is to think about cameras.

Imagine you are trying to measure how hot something is by how much it's glowing. For example you go to resistive coils and see how bright they get. You can get a decent measurement just by seeing how bright it gets. Now try to measure a mirror reflecting an image of the coils. You'll get a very similar measurement. Is this because the mirror is just as hot? No, you are mostly measuring the reflected image of the coils and not the mirror itself.

It's the same thing with thermal cameras. Metal is very reflective at the wavelengths used to measure thermal radiation. You are essentially looking at a mirror and will not get accurate readings from it in much the same way.