OK, so I have tried a new MB with no resolution to the issue...
It is not offset per se, it seems to be changing multiplier or setting Multiplier with P-State OC that disables down-volting.
An offset will allow down-volting with multiplier on auto. with boost enabled or disabled.
If I set the multiplier and an offset I get down-clocking but no down-volting. Boost enabled or disabled.
If I set a P-State OC I get down-clocking but no down-volting. Boost disabled. (Enabled N/A)