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)
OK so it's taken me a while to confirm this since the number of 1920Xs in my circle are limited (most went for the 1950X) and the number of 1920Xs that have tried a P-State OC...however I can confirm that there is something wrong with BIOS or AGESA that means 1920X P-State OC does not work whereas 1950X does. Someone at AMD needs to take a look at this!