When changing the task status to completed from the actuals tab via edit tasks you cannot change the task status again.
When all the tasks on an order are completed, the order is automatically completed. You cannot change the Task Status of a Task on a Completed Order.
If you want to change the Task Status (Actual) on a Completed Order Task, then you need to change to whole Order Status from Completed to something else like Proposed or Released (select Orders, Works Orders). If the Order is not Completed, then the Task Status can be changed from Completed to something else, like Not Ready.
This is how the system works and has not changed; this functionality has been tested on numerous Orchestrate versions (since 7.60) and they all act in exactly the same way as described above. Therefore this is not a code issue.