Groups reverting back to actions

After transforming a task into a group by adding sub tasks, there is sometimes difficulty reverting it back to a task when the visible child tasks have been removed. This takes the form of a bolded task with a drop down arrow which has no purpose and nothing within it.

I understand this is because there are older recurring completed tasks still stored within the task, but should they be discontinued is there any way to have the task simply revert back to its simple correct appearance?

Example with “Proof task” below: