Markers not showing on maps

We are in the process of upgrading our old maps (the depricated extender) to the new maps component.
We followed the very cleas instructions from Kevin:

We are at a point that the map is showing (Google maps because I couldn't find any xyz url options for Mapquest) and the set initial location is working way too good because that's all it ever shows 😢

Our setup is as follows:
 

The address_type_id domain is set as follows:
 

The location data is stored in the correct JSON format:
{ "CoordSets": [ [ { "Lon": "5.9794453", "Lat": "52.2083719" } ] ] }

So the complete data (sort of anonimized and the expression content is added) is:

The maps component is configured as follows:

In the Windows GUI the initial location is shown without any marker:
 

Any ideas/suggestions on how to troubleshoot this?
Is there a way to see what information is passed to the map component?
I also noticed that the marker shown in the legenda and layers are different from the set marker.