3ds Max 2018 update 4.
ProRender 2.2.348 (core 1.297)
When using multiple GPUs and the Render Elements feature, the individual render passes come out black.
Unrelated to this issue, when you first add default ProRender Render Elements to the 3ds Max, the default naming convention uses a ":". This makes the file name/path invalid and causes the render to fail.