Thank you very much for your help. I put the following code inside my view:
{% for post in relationships.veranstaltung_to_karte.from %}
{{ post.osm_karte.latitude }},{{ post.osm_karte.longitude }}
{{ mb.rwmb_meta( 'post.osm_karte', '', post.ID) }}
{% endfor %}
But the output is the same as before.
