Happening to me too, I just started using the Nothing Screen time widget yesterday.
It worked fine during the evening, showing the correct time. This morning it showed 7:38 used time (The same as the time of the day when I saw the phone). I thought maybe I left some app running in the background during the night.
Right now it is 10:04 in the morning and the phone only shows 8:29 of use, so something got weird in the night definitely.
I’ll keep checking and testing and I’ll update this post once I have more feedback.
I’m using Phone 2 with 2.5.2 NOS.