I analyzed the data and it seems in this case the GPU Memory Junction temperature is not reported by the drivers. I'm not sure whether the value shown in GPU-Z is really the Memory Temperature.
Did this value disappear after NVIDIA driver update? Can you try a previous driver version?
It seems to me that NVIDIA intentionally removed reporting of GPU Memory Junction temperature in the drivers, which was somehow expected (they don't want users to know this).
But I'm not 100% sure about this yet.
Can you try older driver versions ?