I would think this is a common issue easily solved with a quick Google search but I couldn’t easily find a simple solution. Tried both note performer and halion sonic. Same issue. Dorico does not play back the ritardando. Ritardando was inputted using shift+T , then typing the ritardando command.
Without inspecting the actual document, we’ll never be able to tell. I can guess that perhaps you’ve turned on fixed tempo playback in the transport? (I.e. is the little quarter note highlighted in blue, or not, next to the tempo value?)