Similar to the curve based text rendering issue in RPR where the text is rendered totally invisible the same thing happens when mBalls are used as objects in the scene.
Please see sample file attached and sample outputs below.
This was created in the latest nightly B2.80 on macOS. It doesn't not matter which rendering device is used. The output is the same. Invisible mBalls.
RPR v2.0.32 output of the scene:
Expected output (using EEVEE):