It seems there is a bug which breaks the spin down if a slot is not used. After re-assigning all disks through New Config without leaving empty disk slots, the spin down works as expected.
before (only the parity disk spins down, disk slots 5, 7 and 9 is empty):
after (all disks spin down, no empty slots):
Why I left them empty: I planned to extend some shares by adding new disks without putting them on the bottom. So it was only a "visual thing" for me.