Apple Watch Ultra 2, watchOS 27.0, Xcode 27. I use CLLocationManager.startUpdatingHeading(), headingFilter=1 and CLHeading.magneticHeading directly, with no offset or GPS course.
During surfing, my app and Apple Compass both showed an apparent error of about 180 degrees. The iPhone was not nearby or connected. Later, outside water, readings returned to normal.
An analog compass on the opposite arm differed by about 30 degrees initially, then agreed. All saved Int(headingAccuracy) values were 89; it still displays 89 while the heading is correct. Raw fractional values were not saved.
Is this an expected or sentinel value on watchOS? What calibration and diagnostics are recommended for the intermittent reversal? The app also uses an underwater-depth extended runtime session; its errors have not been linked to the heading issue.