North of Fatu-Hiva our course changes for quite a long distance into pure E before it returns to the course we really sailed. I observed the error only today, thus I can conclude that it had nothing to do with the position data the IridiumGO device sends automatically every hour.

Please just neglect the PW error and consider our course a straight one. The track as it is logged in the Axiom chart plotter also shows a straight line.