I had the same problem (currently evaluating DrivePool version 188.8.131.527).
After watching drive activity with filespy it seems like some interaction between DrivePool and the WMI Provider Host (WmiPrvSE.exe) is keeping the drives awake.
To circumvent this, I changed the config file (C:\ProgramData\StableBit DrivePool\Service\Settings.json) to disable BitLocker support, like this:
Now my drives sleep the way they should.
PS: Just noticed that this was quite the necro. It is however one of the top search results when looking for 'drivepool spin down', so I don't feel too bad.