Hi Jan,
Whilst you've already found the solution for your issue, for future reference there is one other scenario where this can happen. There is a Project Setting called 'Summary Completion Mode' which has the same result if it is set to Manual. Here is the explanation from Workfront documentation:
"This field impacts how subtasks and parent tasks relate to each other. When set to Manual, subtask completions will not roll up to parent task completion. The manager will need to manually complete the parent task. If set to Automatic, when all subtasks are marked complete, the parent task will automatically be marked as complete. This field addresses relationship between parent and children tasks."
Regards, David