the only place in your model that the temperature can leave is through a face that you've set the temperature? or do you have convection applied to the model somewhere? remember that if you don't apply a bc to a non-conduction face, it is assumed insulated.
have you created a simple stackup example of blocks to check that its not your model and/or the software (depending on what the outcome is)
I had several issues that I corrected to make the study more accurate.
The main issue was that although I had the right material selected for the model, somehow the material of the lower performance thermal pad was changed in the thermal study to Aluminum instead of thermal pad material. That will definitely improve the heat transfer!