Ray Depth. RPR for Blender has some nice ray depth settings just like any other rendering engine. However, they refuse to go above 8. They will play nicely from 1 to 8, but any additional rays are ignored after 8.
Max Ray Depth of 1 on all settings:
Max Ray Depth of 4 on all settings:
Max Ray Depth of 8 on all settings:
Max Ray Depth of 16 on all settings:
Hmm. Exactly the same as 8, but still full of nasty black patches. I am not sure if this is a feature request or a bug report, if the former then additional ray depth really really needs implemented or if the latter it would be very very nice to have it fixed. Behavior is exactly the same on CPU, GPU, or both together. Scene file is attached.
2018 Mac mini i7-8700B 32GB
Razer Core X eGPU with AMD Radeon VII 16GB
macOS 10.15.03 Catalina (issue has been reproduced on 10.14.5 Mojave)