Update: I have managed to get rid of them.
Scenario - All the tasks affected were recurring, had a due date and a due time. They have also been part of a separate notebook
To resolve the problem, I have set the due date for today, removed the "due time". After I have done that they were still re-appearing but without the due date and "overdue" mark. next, I went inside the notebook, and I deleted them from the notebook. That seemed to have fixed it.