Direction, correct mate, also I agree on the assumed behaviour of the backend. The docs only mention lon/lat/timestamp in the proprietary tags, but there seems to be more. And even more these tags seem to be the preference.
I found the app to be very inconsistent and downright wrong when it comes to image direction, in both compass and even software-choice (!) scenarios, since at least 2018.
my numerous emails to the support team failed to uncover the root cause, and the only option that seems to work when capturing using the app is using the interpolation from heading option (also buggy but not as much)