Well, this is annoying. I know I set the Radio button ON before I exited the Dynamic Cut Plane popup menu, but I went back to make sure there wasn't another setting to activate and I saw that it was Inactive. I set it to Active again and returned to the main window, where object was now properly displayed.
That's just weird.
Anyway, thanks for the help, Torfinn.
TC Pro Platinum 2017, 2016 & 2015 (all with LightWorks) & V21
System: i7-5820K @ 3.30GHz, ASRock X99 Extreme4, 16GB DDR4-2133 RAM, Gigabyte GTX 970, Samsung NVMe SSD 950 (256GB), Windows 7 Pro (64-bit) SP1