ActivityMonitor.getInfo() idles

I am calling it from a timer every second.

Maybe you should open a bug report for this. Maybe Garmin will confirm whether this is an intentional design decision or not.

It works fine for me in devices apps.

For example, https://apps.garmin.com/en-US/apps/116a5b59-29ae-4397-a70e-907d7e5f8e44

I suspect it's something that's app specific.

Are there cases where you stop the timer of does it always run when your app is running?

The timer is set like this:

class Controller {
	
	hidden var _fitManager;
	hidden var _myView;
	
	function initialize() {
		_fitManager = Application.getApp().fitManager;
		_myView = Application.getApp().myView;
		timerUpdate = new Timer.Timer();
		timerUpdate.start(method(:removeInitViewAndStartUp), 1200, false);
    }

    function removeInitViewAndStartUp (){
    	WatchUi.switchToView(_myView, new MyDelegate(), WatchUi.SLIDE_RIGHT);
		timerUpdate.start(method(:updateEverySecond), 1000, true);
    }
    
	function updateEverySecond(){
		_fitManager.updateFitData();
	    WatchUi.requestUpdate();
	}
	
	// ...
}

Within fitManager.updateFitData:

class FitManager {

	hidden var stepsLast = 0;
	
	// ...

    function updateFitData(){
        setStepsLast();
	}
	
	function setStepsLast (){
    	if (ActivityMonitor.getInfo().steps != null){
	    	if (ActivityMonitor.getInfo().steps > stepsLast) {
	    		stepsLast = ActivityMonitor.getInfo().steps;
			} 
			else {
			    // !!!
			    // This is called if ActivityMonitor.getInfo().steps did not update
			    // or if indeed no new steps were counted.
			    // !!!
			}
	    }
    }
	
	// ...

}

Garmin does some interesting things with steps - for example "10 or more" on some devices,  If you take 9 steps and then stop, you may not see a change.  It's a way they reject arm movement without talking a step.  Make sure you are walking enough at one time for the data to change.

If you are just testing at your desk, you may not see any change.

Understand that with all that's involved, you may only see steps update a few times a minute.  You can verify this is you have a WF that uses onPartialUpdate and use it for steps,

Here is an example fit file record of my app where I saved the time between steps during an activity: 

https://services.garmin.com/appsLibraryBusinessServices_v0/rest/apps/03644288-d83e-40f3-a7cf-a9c7b0eec6e1/screenshots/232d3a3e-9f85-4a1a-b1c4-e6adc88e159c

Whenever no new steps where given by ActivityMonitor.getInfo().steps, I increased the time counter. 

As you can see, there are several spikes where my counter kept increasing. In fact, most of the peaks show that I looked at my watch during the activity...

I have tried your app today. In fact, also your step count does not update all the time. It tends to pause sometimes - similar to the observation in my watch app. I guess that this is rather an issue with the ActivityMonitor then. I don't know if it is watch related though. I will ask my friends to use my app as well to see if they can reproduce the behavior (similar to https://services.garmin.com/appsLibraryBusinessServices_v0/rest/apps/03644288-d83e-40f3-a7cf-a9c7b0eec6e1/screenshots/232d3a3e-9f85-4a1a-b1c4-e6adc88e159c )