It looks like your mapping is working, because it has generated the "[]" and it is being sent, so your Content-Type is also correct.
Check with the debugger that you actually have data in your list.
Another check you can do is to manually export your list through the mapping to a string so you can see this in the debugger too.
Hello Jelle and Colin,
Thank you for your response. Yes CountObservations has the value 2, but I can't see the values of the list-entries in the variables-tab: