I have the similar issue on my Ultra 2 since watchOS 26
Sitting and reading the newspaper on my iPad after breakfast is seen as a nap, as well as getting to bed early in the evening, just because I'm sitting in front of the computer streaming content, without hardly moving in my chair.
Or checking the time on my watch in the night, turning around after that and falling asleep right away is counted as being awake for 20-60min.
watchOS 26.2 showed some improvements for two nights and then continued to give false readings.
My 3rd party sleep tracking, which uses the same data from the Health App, is showing more realistic results.
All effected users should all let Apple know about this issue by using the Feedback link, I have already submitted mine.
Feedback - Apple Watch - Apple