That’s the HDR kicking in! The show may be more dim, but it will have much more accurate colors and you are viewing closer to how the director intended.

That's an unfortunate shortcoming of our current displays and current standards.

The standards are ambitious and allow for dynamic ranges well beyond what our hardware is currently capable of. Even our most expensive displays can't do the full range of our standards. So we play games with mapping of the theoretical range to our comparatively dim displays. Even after doing that, many people still find the image annoyingly dark.

You may find that you enjoy watching HDR content converted to SDR. It will be brighter and easier to see, though less accurate.

HDR displays are getting brighter each year. But for the foreseeable future, HDR content will still appear too dim for some people. There's no way around that other than brighter displays or a standards/usage change by the industry.

Personally, I think HDR could have been handled better by everyone. Someday, our current content mastering will display beautifully. It just doesn't quite yet.