Why tasks that have been set to be free of charge still are displayed under "Pending invoicing"

Answer

By default, all tasks where either time or item consumption is registered will be displayed in "Pending invoicing" regardless of which invoicing method is selected.
This is done so that you have the opportunity to review the tasks in order to get a final confirmation that they do not need to be invoiced, as hours or consumption of goods have been added.

Be aware that the automatic invoicing of tasks takes place at midnight and you can therefore experience seeing tasks during the day waiting to be handled.

You can automate this process by choosing an automated invoicing strategy.