Intel Rapid Storage Technology write-back cache keeps disabling itself

On several occasions now, I have enabled the write-back cache, only to find that later it is disabled again.

This is on Windows Server 2012 on a Dell Precision T3400 system with a RAID 5 array, using Intel RST software and driver both v11.6.0.1030. (This is the latest version I could find which will actually work on this motherboard which uses the ICH9R chipset, as Intel has so helpfully removed support for this older chipset from the newer versions of the RST software).

Before anyone asks, yes, this computer is plugged in to a UPS.

Why does RST keep disabling the write-back cache on this array?

thanks,

Jay