I suppose that you can't tell the difference. But, it's far more likely that if the Go App map heading indicator doesn't agree with the actual orientation it's because the launch site is geomagnetically distorted. The only way to fix that is to move the launch site to a location where the heading indicator agrees with the actual orientation. If you can't find such a location then it's time to think about re-calibrating.Or, that the compass is broken.......
Being very new to drones, I'm keen to learn. You said the way I did the compass re-callibration wouldn't have helped. How do you tell if the compass just needs re-callibration because it's been in the car too long or otherwise and when it actually doesn't need re-callibration because the discrepancy between the way the drone is pointing and the direction the Go 4 app says the drone is facing is due to magnetic interference? I'm guessing you would walk to a nearby area that is most likely free of metal, such as a grassed area and see if the two agree there.
No amount of calibrating can "fix" the problem of a geomagnetically distorted launch site.
You said that you had flown in this area before without incident. The distortion extent can be small - less than 2 cm. So moving the AC 1 cm can make the difference.
BTW, in the incident you reported the Go App would have been showing a heading of 75° at launch but the AC actually had a heading of 30°.