Mine behaves the same way. I finally figured out that because my Time Zone is determined by New York, where Sunset is one hour earlier than where I live, in Atlanta, GA, that hour isn't going to change as long as I have the Automatic switch turned ON. I tried changing the setting to my actual location and as soon as I set the Automatic switch ON, it changed back to NY and the earlier hour. It doesn't matter if EST or DST, the earlier hour is triggered. My son lives in NYC and we were discussing this issue yesterday on a Thanksgiving FaceTime call, when Dark Mode turned ON mid-call.
Then again, anecdotal evidence is all that I have to go on. It's annoying anyway.