Using the preset: “Optimize for two voices” did it.
Unfortunately it does not allow you to turn off the voices.
Run Cubase in safe mode ( plugin and preferences dissabled):
- Convert a drum staff to polyphonic
- it places a grand staff with Bass clef
- Open the score setting and try to uncheck this voices. It’s nothing there to uncheck.
I suppose that is another bug on my side besides the display glitch, if I am not mistaken.