Thanks for the more detailed explanation, now I understand what the problem is!
When you add a tag to a task ("tarefa"), this tag is only being added to new work units that you created from then on. It is not added to past work units (like the one in Aug 1st). At least in the German and English translation it also should say that it only is added to new work units in the app.
Ricardo Back The Working Unit does not have a field to assign a tag. I can only assign tasks to a working unit, right?
No, you can add tags to work units at any time. You can either right click in the main list:
Or you can use the "Add tag" button in the details page:
You can also add tags to old work units very quickly by using "group by year" in the settings, then filter by the task you want to tag and then using the context menu on the grouping year to add the same tag to all filtered work units of that year