Autopilot heading display units can't be reset

[sandy11]

I have a Raymarine ST70. I had a constant disparity between my Ritchie compass and the Autopilot heading displayed of about 25 degrees. I have been able (by removing a metal step and multiple other items around the fluxgate) to narrow this disparity to 16 degrees. My E120W chartplotter and the AP are both set to display Magnetic. Because the variation here (Seattle) is about 16 I decided to check the settings in the AP. It was set to Mag so I reset it to true to see if the disparity would double. After the reset I observed the AP heading and it remained labeled as Magnetic and the numbers had not changed. I rechecked the units in the setting mode and confirmed it was set to true. When I backed out again the display remained as before.
Is this a known failure mode or does it suggest some other error? I am about to undertake a lengthy trip and would appreciate having an operative--and trustworthy-- Autopilot.
The COG on the E120W and handheld binocs with compass reading both agree with the Ritchie numbers