Export Mapping excluded items

When importing a JSON structure into an Export Mapping, I am not able to actually select the attributes that are in child elements.  In the images below it is visible that a valid JSON structure is used and the mapping shows that elements of child objects are greyed out.
1 answers

I just tested this in 6.9.1, and in that version it works as expected: I can select objectId. Furthermore, this is explicitly mentioned in the release notes of 6.8.0, see https://world.mendix.com/display/ReleaseNotes/6.8.0