I don‘t think it has anything to do with software versions as mine has worked since day one and I’ve had 2 updates since Nov.
You said you had auto dark mode selected in settings. Did you uncheck that and try manually adjusting or do the adjustments stay greyed out? I’m thinking you have but just asking.
I know it’s a pain in the a$$ but have you tried a factory reset? (I think it can be done from settings) If you have I’m assuming again that didn’t help either. If not then it’s something worth trying.
As you are, I’m at a loss why it won’t work.
Thought I would mention the other forum incase you didn’t know. Glad your over there also.