If this is the issue then you should be aware of a work around. GPS heading only works if the GPS is moving, so keep moving. If you are going to stop, you can check the direction before you stop. Also, be aware that it only tells you the direction of motion. If you are facing north, but travelling east, it will say east because it assumes you are facing the direction you are traveling.
It sounds like a simple code change (very simple to just skip the test that makes it use GPS sometimes), but then there is how to build the code, so best done by someone who is already familiar with this.
For CC3D, code could check the speed as reported by the GPS and only use GPS heading if speed is above some small value and use Attitude state with offset when below that speed. CC3D yaw heading drifts and does not correspond to north, so when it is moving it says aha, offset from north is 23 degrees and when it stops it uses Attitude state yaw + 23 degrees. This doesn't fix the underlying issue where pointing north and flying east, but it fixes the jumping around.
Code could check which FC it is running on (e.g. look for any of several telemetry packets that only come from Revo class FC's) (and/or what AttiEstAlgo is running) and always use Attitude state for Revo class FC's (or GPS class AttiEstAlgo).
Simplest yet would be to have two versions of the code, one for CC3D and one for Revo.