The task's CorrelationToken's OwnerActivityName decides the scope in which the task is controllable. When a workflow is in the same state as that of the OwnerActivityName state, the task can be modified/deleted safely.

If the OwnerActivityName is set to a state that is not the task's parent state, or is set to the root StateMachineWorkflow state, modification and deletion of the task may be locked by the workflow. Generally, the task's lifetime should only be in the scope of it's Owner state.

If a scenario arises, where a task has to be completed in another state, consider changing the state to the task's owner state, modify/delete the task and then jump to the necessary state.