I agree we can discuss this outside discourse. I will reach out soon.
I will also make sure to update this post when we find a fix so that others following will be notified.
So the problem turned out to be VTK_USE_X=OFF. When that is OFF, the cmake goes ahead and declares vtkOSOpenGLRenderWindow the factory override in https://gitlab.kitware.com/vtk/vtk/-/blob/master/Rendering/OpenGL2/CMakeLists.txt#L371-L375
That code is the old way VTK worked with OpenGL and needs removed.
I hope we can fix it in the next RC.
FYI @cory.quammen
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I stumbled unto this but assumed this was intentional for some reason. good find.
Congratulations and thanks to Jaswant Panchumarti who demonstrated his expertise during our live debugging session. Well done!
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And many kudos and thanks to @jfavre for testing ParaView 6.0.0-RC1 as soon as it was tagged! 
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