Is the PCB in use or is it off?

The operating temperature of any PCB is going to be higher than it’s environment because the electrical currents create heat through the part. So if you’re measuring 105C at a powered on PCB, then I would argue the chamber itself is not actually 105C, but lower.

When you say the chamber was set to 105C is it regulating the temperature with a thermocouple that is actually measuring the temperature? If so where is that thermocouple? And where is the heat source? Is there one heat source or multiple? Is the chamber being heated by driving heated air through it? Also how insulated is the chamber?

It seems likely that the air in the chamber isn’t actually 105C. This could be for a lot of reasons, like if the chamber isn’t insulated very well or if the heating element is too small or the heat transfer from the element to the air isn’t very efficient or if the feedback loop that targets a temperature setting has a bias from poor temp sensor placement or calibration. Any or all of these things could lead to inaccuracies in the chamber temperature. And it’s very likely that you don’t actually reach thermal equilibrium no matter how much you “soak” if the chamber is losing heat energy, which it almost certainly is.

Think about a 3D printer, the bed temperature is regulated with a thermocouple that is attached to the bed. If you set that to 105C bed temperature, the air in the enclosure is absolutely not going to be 105C no matter how long you soak for. Because the thermal losses from the chamber are too great.

My guess is that this is what’s happening to your chamber. I’d put an old fashioned thermometer in there with no PCB and see how close the air temperature is to the setting after a 30 minute to one hour “soak”

Edit: if you want to tell me more about the chamber (ie answer some of my questions about how it’s heated) I can help get more specific. Also knowing what the chamber exterior is made of would be helpful.