thank you for the fast answers.
Of couse the drives have to spin up to stop the array, no problem with that, but after the OS finished all the operations to the disks the disks should park their read/write heads and shutdown properly.
If they would do that the SMART value "power-off retract count" wouldn't increase.
If i manually spin up and spin down the disks in the unRAID-OS the drives are shutting down properly and the SMART value doesn't increase.
I don't plan to shutdown the whole system very often but all my other systems (Windows 10/7, Linux Mint, macOS) aren't causing an increasment of the particular SMART value mentioned above.
Could one of you maybe check the value on one of your disks? That would be very nice