Today I discovered, that in my FIT files the entry "max_heart_rate" was set to a different value, than the one I entered on my watch recently.
The value recorded, was the one, which was autocalculated, after I set the HR zones to lactate threshold in the GCM app.
So, my guess is, that, as with the alarm settings, the app overwrites the values entered on the watch. To make things more confusing the watch still displays the value entered on the watch as the maximum heart rate, despite all FIT files showing the value from the app.
My workaround for now is to enter the correct values on the watch _and_ in the App and correct the maximum heart rate value, which was automatically calculated at the bottom of the lactate threshold HR zones.
It might have an effect on all the performance measurements like VO2max and lactate threshold.