Thank Neil. I did reach out to Netflix (first thing I did) and obviously am aware of the note about "may not be available at all times." I usually post on the forums as a last resort, so I've typically exhausted all the obvious avenues.
At any rate, an update if anyone is looking for solutions:
It appears that Netflix doesn't always do a good job of detecting whether or not your computer / browser are capable of Ultra HD (UHD). So, while browsing, it will often show the plain old HD logo instead of UHD. Recently, I've been seeing the UHD logo sporadically which indicates that perhaps this is something they are working on. Somehow, the website needs to detect that a) you have the T2 chip, b) your monitor is running 60hz and c) that you have a fast internet connection.
HOWEVER: when I watch a movie, I'm able to see if it is indeed streaming in 1080p or in 4k. How? By using Netflix's super-secret keyboard code Shift-Control-Option+D which shows statistics about the currently streaming movie. If you look about halfway down the matrix-like gibberish (to me at least), you'll see "Playing bit rate" and it will show the resolution of the current viewport: 3840x2160 if in 4k. 1920x1080 if in HD (or lower). Now, I'm not exactly sure what this signifies: is it the resolution of the currently streaming video? Or is it the resolution of the screen it is streaming to? Is the content being upscaled (or downscaled)? I think it is showing the actual resolution of the streaming content because a) when UHD resolutions are shown, the quality is very good and b) the resolution shown will often vary depending on the movie (older/newer, etc.) and on how close / far I am to the router and my throughput. Sometimes the resolution will change mid movie if my connection is week.
Generally, it appears I'm streaming UHD movies in 4k regularly as the statistics show 3840x2160. And when watching an HD movie (older content) it shows 1920x1080.
So, in summary, if you're having this problem, take a look at the actual stream statistics (Shift-Control-Option+D). You might be getting 4k content even though in browse mode, the logo shows as HD. Until Netflix fixes this, it seems that's the best we can do.