The defect in question had one task associated but no test cases. The defect was parented by a user story which had many test cases associated to the user story. The user story was cleared as a parent and then tasks were able to be created under the defect. The user story was then re-parented to the defect and tasks were still able to be created. The user story was over 2 years old and had many test cases associated to it. It is likely that some of the associations needed to be refreshed or re-established by un-parenting and re-parenting the user story to the defect.