SSD temperature is static number?

[shengyang998]

In my case, it's fixed at a range of 69°C to 71°C no matter how much data I write or read from SSD.
But if the HWMonitor is restarted, the temperature will change and then hold still.
Is it a bug?

System: macOS High Sierra, with APFS.

Here is a test. In this case, I copy a 7 GBit data from external driver to SSD, and it seems like
Screenshots:
initial:

restart the HWMonitor and start to copy. The temperature is fixed at 69°C :

data is copied, and then restart the HWMonitor. The temperature is 73°C :