Watchface: Activity.getActivityInfo().currentHeading freezes on Venu 2?

Hi Community,

I try to make a Watchface for Venu 2(S), which uses the compass/heading. Using Activity.getActivityInfo().currentHeading works great in simulator, but on my Venu 2S (v5.08) the radians stop updating after a few seconds and never update again. I am looking for a similar behaviour as the "orange" original Garmin watchface (see image), which seems to be able to use "currentHeading" all the time with about 1Hz samplerate without freezing. 

This is a minimal setup to showcase/reproduce the issue. I use the "Default SDK 4.0 Watchface"-Project and Update the "View.mc" to this: 

import Toybox.Graphics;
import Toybox.Lang;
import Toybox.System;
import Toybox.WatchUi;
using Toybox.Sensor;
using Toybox.Math;
using Toybox.System;

class testView extends WatchUi.WatchFace {

    function initialize() {
        WatchFace.initialize();
    }

    function onLayout(dc as Dc) as Void {
        setLayout(Rez.Layouts.WatchFace(dc));
    }

    function onShow() as Void {
    }

    function onUpdate(dc as Dc) as Void {
        var clockTime = System.getClockTime();
        var t = Lang.format("rad $1$ \nrand $2$", [Sensor.getInfo().heading, Math.rand().format("%02d")]);
        var v = View.findDrawableById("TimeLabel");
        v.setText(t);
        View.onUpdate(dc);
    }

    function onHide() as Void {
    }

    function onExitSleep() as Void {
    }

    function onEnterSleep() as Void {
    }

}

To make sure its really only the "currentHeading"-variable that freezes, I print a random number within the same textbox that I update (feel free to replace for clocktime-"seconds"). When running this in simulator, both lines update once per second - like expected. When running this on my Venu 2S,the random number updates every second, but the currentHeading instantly freezes or freezes after a few seconds (couldnt find any pattern yet). 

What am I doing wrong? Is the source code for the orange garmin watchface public? 

Thank you for your help.