Personally, I’m far less sure that is the cause.
Over the last 5 years of drone ownership I’ve driven to Italy, and around Italy, and back, twice … once with my Phantom 2, and more recently for 3 months with my MP …. and around the UK with both.
I recalibrated the compass on the P2 just once, when I got it, because it said you should.
As yet, I’ve never recalibrated the compass on the MP even once.
The change in magnetic declination in Europe is too little to make much difference wherever you are.
On drone/DJI/phantom/MP forums, over those 5 years, I’ve read a good many reports of flyaways and come to my own conclusions …
a) If it flies OK the first time … don’t fiddle with it
b) the more times it’s recalibrated, the more chance there is that one of the calibrations goes wrong, and you’re worse off than not having done one.
c) I’d look at that chart if I’ve travelled much further, and decide whether to bother with a recalibration.
Indeed, the compass accuracy is needed whilst in flight. If you recalibrate where there is some very localised warp in the magnetic field on the ground, once it’s flying and away from this anomaly you’ve basically just built in an error.
Interestingly, of all the people I’ve known that virtually never do a compass calibration, none have had a flyaway, and of the ones that I have read about that had a flyaway, most are reported with " … I did the usual recalibrations before the flight … ".
Personal opinion, and I’m sure there’ll be exceptions and those that want to disagree, but … if you understand what it is you’re recalibrating, why/when it “might” be necessary, and the limited impact one that is slightly out of calibration can have on a flight … you’d probably reach a similar conclusion to myself.
I realise that this doesn’t bring back your MP and, should you follow them, my thoughts mean you’re no closer to understanding what DID happen.
Believe me … I’d really like to know the real reason, if it can be found an proven, even if it means I have to completely change my thoughts on the compass recalibration issue.