I ran into the same situation, when building a rackspace with my Arturia B3. Everything fine: drawbars etc. are mapped and responding. However when I change a Preset in the B3 plug-in the GP Drawbars etc. do not get updated?!
As a workaround I figured out, that I can trigger an update by selecting Preset01 in the GP Plugin Manager?! The Arturia preset stays the same, GP drawbars get updated and everything is fine.
So basically there is an update mechanism, but no dedicated (manual) sync…
BBB