Thank you CatBus. Your explanation makes sense. I did try muxing in the 1080p SUP files, and the results are good upscaling to 2160p/24. 1080p subtitles also work properly for the 2160p native 4K77.
Off topic, but I also discovered that the UDP-203 apparently doesn’t like more than 10 audio tracks in an MKV file. If your MKV has 11 or more, I will only “see” the first 9 and then the nth one.