Okay, the attached file contains a very simple action bar addon that is movable with a right click so it isn't in the way.
All it does is force a particular page on it so the buttons will remain empty until that page is available. I tested this with the vehicle bar index on the farm weeding.
The block of code that affects what is shown is as follows around line 34 :
Lua Code:
-- As a standalone Action Bar
-- The below action page is used in instances like Drak'tharon Keep
--actionBarFrame:SetAttribute( "actionpage", GetTempShapeshiftBarIndex() );
-- The below action page is used in little games like at the fayre
--actionBarFrame:SetAttribute( "actionpage",GetOverrideBarIndex());
-- The below action page is used on the farm
--actionBarFrame:SetAttribute( "actionpage",GetVehicleBarIndex());
If someone that is having this problem in nUI can give this a quick test through it would be appreciated. I suspect the OverrideBarIndex is the correct one looking at the video clip I posted so try that one first and see if you get lucky.
Simply take the -- off of the beginning of 1 of the SetAttribute line you want to check, save the file and reloading the UI before starting the scenario quest from the quest giver. Do them in turn a couple of times to make sure only 1 of them works (hopefully) and let me know which one always works and displays the buttons and allows you to use them properly. While you wait for a fix this can then be used in the interim, hopefully.
I'll then work out a simple way for people to test the scenario again to auto trigger the correct page based on a macro conditional on a main bar rather than a special bar of its own.