understood,thanks kat.
although i'm suspecting in this case it's more of a bug specific to blender, rather than gfx drivers etc, because i've just noticed if i join two of the viewports then split them, the widget is displayed correctly. also it's not as random as i previously thought, as the bug appears to be in the same viewports after saving and reloading the .blend file. at least it's straight forward enough to fix, even if somewhat annoying.