For practical reason I would like to have a player with a few unpitched instruments (drums and percussions) and a keyboard.
There is no problem adding several types of instruments to a player in Dorico as soon as I try to remove a staff from the grand staff keyboard, Dorico shows all instruments in the score instead of the keyboard only.
This behaviour can be easily reproduced (at least on my system):
Add a drum Set, any other percussion instrument and a keyboard to one player.
Write some notes in the upper staff of the keyboard: as expected Dorico will show in the score the keyboard grand staff only.
Remove the lower staff of the keyboard: Dorico now shows all three instruments in the score (drum set, the percussion instrument and the keyboard with one staff)
Is it a bug or is there a reason for this behaviour?
It’s a limitation: once you start adding or removing staves, then hide empty staves no longer takes effect. This is partly because we anticipate that you are telling Dorico that you want to manage which staves are appearing yourself, and partly because of the complications of handling all of the possible combinations. For pragmatic reasons sometimes we choose not to chase all the way to the bottom of the rabbit hole if we judge that a specific combination of settings is less likely to come up in real-world examples. Comparatively speaking few users have run into this limitation, so I don’t feel too bad about it.
In the meantime I suggest you use two players – one for the piano and the other for the percussion instruments – and assign them together in the same layout.