For OpenGL I can confirm what you found. If you put in anything other than 0.0 it will change to that, but at 0.0 the clearance in the editor/browser goes to 0.2 mm.
The 3D viewer depends on the settings of the tool that started it.. it shows what one sees in the respective views.
PCBnew works correctly as you found.
For Legacy mode the solder mask clearance doesn't show up at all anymore (if it ever has, can't remember).
Anyhow, feel free to chime in, just filed it: