k-type thermocouple with latency
Thermal lag of the heat stored in the oven's steel thermal mass taking time to transfer to the air of the steel in the oven. That's normal and expected. Your PC does that too. Even if you're in direct contact with the oven, heat takes time to flow from the hottest steel parts to the cooler steel parts. If your thermocouple is not fully embedded into the steel, then the surrounding air also has a cooling effect so long as it is cooler than the surrounding steel (i.e. until it reaches the same temperature of the steel).

That's why some devices demand you not just cut power to them. You have to turn off the heat and leave the cooling (usually a fan) running for some time before turning it off or else you destroy it from overheating.