Mendix support gave me the solution:
Not all one-to-one association values are communicated correctly from the Mendix Runtime to the Mendix Client. This could lead to values that seem to be missing in views, but which are present after a reload.
Workaround: Explicitly commit autocommitted objects in a microflow to ensure the results are correctly sent and shown
Committing the list of objects given to the Chart Widget solves the issue.
Seb