OK, thanks Avechelice. The problem seems to be that a minor error in the clickspot coordinates for the heading button is causing it to overlap the heading decrease clickspot.
In the HJPGauges folder find the .xml file for whichever autopilot derivative you're using and drag it into an open Notepad window. (i.e. if you're using "gauge00=HJPGauges!AUTOPILOT AMBER" in the panel.cfg then open "AUTOPILOT AMBER.xml")
Scroll nearly to the end of the file and look for the code below:
Simply change Height="206" to Height="20" and save. Note that you won't be able to save the xml if the panel is loaded in the sim so either select an aircraft that doesn't use the autopilot or close the sim to make the edit.
<Area Left="44" Top="4" Width="34" Height="206">