Hot air doesn't cool. Thermal cameras and ir thermometers give invalid readings of reflective surfaces. These are two facts that answer the op's question and they are both relevant to the scenario. I don't really have anything else to say because there isn't any more to say about the scenario.

Very cool. So many early (and even more modern types, like inertial and star-based navigational systems) measurement and navigational tools rely on extremely simple and primitive concepts or operations, but they produce such useful data.

The metal housing wasn't the oven's housing, it was the housing of the part being heated, or at least that's what the OP typed grammatically. They may have meant something different. Either way the temperature delta wouldn't have anything to do with the heated air, it would just be thermal conduction if it was the interior of the oven wall that was cooler than the internal ambient air. Also, you can test this on your own very easily. Go try to use a thermal camera or IR thermometer on a shiny piece of metal. You'll receive obviously inaccurate readings. Place a piece of dark tape or paint on said metal and watch as the temperature is more accurately represented.

Edit: I'd also like to point out that this is inside of an oven. There is more heat being introduced to the system on a consistent basis to maintain a constant-ish temperature. If there was a hunk of metal inside that was still below the target temperature, it'll keep introducing heat. Preheat your kitchen oven empty and then with a cast iron skillet inside. The heat capacity is many times higher with the skillet vs just atmosphere.

I don't think this applies as this is a nominally steady state environment, i.e. it's temperature is stabilized regardless of input heat required to stabilize the internal ambient fluid temperature. It's just because they're reading a reflective surface with an IR camera/thermometer, thus giving incorrect readings.

Unless some form of phase change surface cooling (evaporation in this case) is in play.

I first experienced this while deployed to Afghanistan. Some Brit showed me how to put my bottle of water into a sock and soak the sock with water from a nearby stream. I'd then swing the sock around for a few minutes just to get the water temperature below the 120+ degrees F it was likely at so that it didn't raise our body temp when we drank it. It would often hit 120+ F ambient air temp and sometimes the water would either be in the sun or on top of some surface that was even hotter than the air temp.