Hard Disk Monitoring

There's a bug in v4.4, when I set the power management value for a hard disk (e.g. 127), it stays for a while but after a few minutes again it resets back to the default value.

Why doesn't Sentinel monitor this value and reenforce it / check it every few minutes?

Hard Disk Sentinel works exactly as you wrote: it re-adjusts the value if required (if you enable that option in the window).

If you still see the old value, it means that either the
- hard disk may reset the value to default (which may happen for some hard disks after a complete power off / on cycle). This is _exactly_ why the option to re-adjust the setting is available in Hard Disk Sentinel.
- the hard disk controller (more precisely its driver) where the hard disk is connected does not forward the proper command to the hard disk
- the hard disk controller (or its driver) provides an older (cached) value when Hard Disk Sentinel tries to detect the updated status

If possible, I'd recommend to check for an updated / recommended hard disk controller (chipset) driver from the Support -> Driver Zone.
You may also use Report menu -> Send test report to developer option, as then it's possible to check the actual situation and advise.