This is a legit issue. I believe it is a bug in the logic.
My 2025 exhibits this behavior - stuck in dark mode (meaning the option to toggle modes is greyed out) after turning lights off - majority of the time. Other times, the toggle remains available after shutting off headlamps. The idea, I believe, is it automatically switches to dark mode for you - a feature which IMO is not needed and nonsensical. I personally want day mode all the time as I find it very difficult to see my screen in dark mode under any lighting condition. I struggle to understand why they did not leave it as an available toggle at all times.
In any case, I have not found a repeatable cause pattern nor a solution. As a work-around, I have been leaving my lights on all the time - so far so good. The natural concern of course was battery drain after shutting down the machine. The headlamps automatically shut off after ~10-15 seconds - so far anyways. Other than turning brights on/off, I have not touched the headlamp dial in a couple of weeks. Best of luck.