Hello, I'm and end user as well and to the best of my knowledge I don't think Google has this listed as a known software of firmware problem. Having worked with these cameras (and many others) for a number of years this can happen when the camera is catching a reflection and it actually thinks the room keeps going from day time to night time.
Since you have 2 cameras (for testing purposes) can you swamp them around and see if the problem stays with the same camera in it's new location? If it still does it in the new location then it sounds like you have a camera that needs replacing. Google will most likely offer to replace it if it is still in the 1-year warranty.
Here is the link to open an official support ticket with the Google Techs.
https://support.google.com/googlenest/gethelp
