Fluxgat Compass vs. Autopilot vs. Chart-plotter

[DWEnstrom]

I accidentally came across what I think is a bit of an issue last summer.

The hardware and situation:
- c95 chart plotter
- SPX-5 autopilot with fluxgate compass

I was under autopilot tracking to a waypoint. I inadvertently placed my tablet near the fluxgate compass. The tablet's cover has some powerful magnets, which naturally threw off the compass. Despite being under autopilot, and heading to a waypoint, the boat did several 360° turns!! I eventually powered everything down and the system seemed to come back to normal.

So the questions are:
1. Why wouldn't the GPS tracking towards a waypoint not take precedents!?
2. What if the compass failed in some way, would the chart-plotter and auto-helm still function!?

Concerned;
David Enstrom