RPR 2/0/85 / Blender 2.80 / Windows 10 / 2x Radeon VII
Reflections and Camera visibility options do not behave as expected if the objects are set within a volume object unless the camera is within the volume as well.
The highlighted mesh is a simple principled volume object and each of the cubes has one of the visibility settings disabled, Camera, Reflection, Refraction, Diffuse and Shadow respectively, with the red strip being a simple transparent plane to test Refraction visibility. Please see scene attached for more information.
The Reflection visibility setting behaves very strangely within a volume as well. The background plane has no diffuse, the bottom one does have diffuse in addition to reflection and the corresponding visibility setting seems to be ineffective. Basically, if the mesh is within a volume object and the reflective plane also has a diffuse property, reflection visibility doesn't work anymore.
It's a very complicated bug to explain, I hope it makes sense.