See under the band’s properties if NaNs are enabled and how they are stored. Sometimes 0 is considered NaN.
It can also be defined in the first window of the band maths already.
maybe the NaN you see in the pixel info are created by an valid-expression or by applying a no-data value.
You can check your band if these properties are set. Easiest to see in Information Window.
What type of product are you using?