Two things make me think it's a compass issue 1) Arrow not pointing in exactly the right direction at start of mission, and 2) recalibratation (after which arrow points correctly) fixes the issue. I realize that's correlational, but that's what I've got to go on. I have a backup archived version of the mission, which I also ran, and had the same issue, so pretty sure the mission hadn't changed. If I had to say if the issue takes place for the whole mission, it looks like the problem accelerates during the mission.