Hello Dr Memo Guerrero,
The cooling figure is a delta. We specify a 38°C delta for the Atik One 9.0, so the camera can cool to 38 degrees below the ambient temperature. So if your ambient temperature is 21°, and the camera is reaching -16°, that is actually a 37° delta, so I really wouldn't be too concerned. However, I appreciate that's still 1° off specification, so please do drop our support team an email on support[at]atik-cameras.com to check everything's okay.
If you also send them a FITs file showing the hot pixels you're concerned about, they can take a look at those too, though it's not uncommon to see a small number on these kinds of sensors.
I hope that helps,