Got a little late to the party...
How did you find out that cockpit was the one preventing idling? I have the same exact issue, but I don't have cockpit installed
the problem is udisks2 package that is installed as dependency of cockpit and when its daemon start, overwrite ata settings of hdparm used by openmediavault.
Furthermore it pool s.m.a.r.t. data from drives regularly.
Hard drives with a longer standby timeout than the polling interval (udisks default to 10 min) may fail to enter standby.
Problem is described in section:
Broken standby timer (udisks2)
Possible solution in section:
Apply ATA settings (udisks2)
I hope I was helpful