It appears that Volumes provide some nonstandard and unexpected behaviors.
We see here that the main domain volume occludes the emission of the fire, while not preventing the text inside from being lit through indirect bounces. There is also this interesting effect where the ground plane is extended inside the volume, but has no effect on the text directly behind it. There are several components to a volume cache, here detailed with Maxon's Cinema 4D:
It would appear that the Shadow component is the section responsible. Rendering out with several AOVs further confirmed this:
Direct Diffuse
Indirect Diffuse
And finally Opacity
Opacity is interesting to me as it shows the Volume with a 100% opacity, something reflected by both Diffuse AOVs, however both Illumination AOVs and the Combined image do not represent this, instead showing the same artifacts:
Direct Illumination
Indirect Illumination
And a repeat of Combined
Cycles is not affected by this Shadow volume. Unfortunately I cannot render out an example of it.
Hardware:
Mac mini 2018 i7-8700B 32GB
macOS 10.15.0.D2 Catalina
Razer Core X eGPU with Radeon VII
Blender 2.8 β & RPR 2.0.95
