OSPRay suppport of interactive 3D widget is quite incomplete/unstable at the moment so this doesn’t really surprise me (see issue 7603 for example). This will hopefully be fixed in a next release.
On the other hand these widgets are not meant to be on the final render anyway so you can just place them using the OpenGL rendering and then switch back to OSPRay for your final rendering. Also this workflow is quite common since OSPRay rendering is not interactive. It is usually better to keep using OpenGL for setting up your pipeline and then switching to OSPRay.
I can’t think to a better alternative right now 