As already mentioned, the temperature issue you are seeing are most likely related to you wearing in on your wrist such that it is influenced by your body temperature.
I mount my Mk1 on my camera rig (which gives me effective heads up display). I have found the temperature sensor to be quite accurate at least as accurate as you could expect when it only displays an integer. That said, for some reason it has been reading one degree C cooler than my Citizen (which I know is accurate) for the last few dives.
Here's a comparison of the logged temperatures (for a dive in November) between my Citizen Hyper Aqualand, my Shearwater Perdix AI, and my Garmin Descent Mk1:
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It was a warm day and the Perdix takes the longest to adjust from the absorbed heat in the air.
Here's a dive from mid-December:
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Here are the most recent three dives where both the Perdix and the Mk1 are 1 degree C cooler.
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