I’ve seen/fixed similar problems in the past where the export was being done from the GUI and the exported image resolution was larger than the view, causing VTK/ParaView to go into stitching mode to make up the large images, which tripped up the accumulation mode we use for progressive refinement.
My guess is that this is happening again.
If you are building ParaView from source you can hardcode progressive refinement off as well here:
https://gitlab.kitware.com/vtk/vtk/blob/master/Rendering/RayTracing/vtkOSPRayRendererNode.cxx#L1170.
Otherwise, please try a smaller image and let me know if this hides the problem.