The black screen with the white KDE logo that appears while the desktop is 1920x1080 on my 4K DisplayPort monitor. My secondary monitor (HDMI) is 1080p, and that's where SDDM shows up for some reason... https://github.com/sddm/sddm/issues/115 If I have my secondary monitor disconnected, the loading screen fills up the whole display and appears to be the correct resolution. If monitor 2 is connected, whether it's enabled or not, the loading screen fills up a quarter of the display.
This seems to be fixed now. Nice! My SDDM issues are almost all resolved too, I wonder if that's related.