How exactly do you complete the task, from the task view, from the 
task list, or from task form? And does it happen in all cases?
Also, what exactly do you mean with "gets lost"? What are the symptoms 
you see?

When completing a task with a parent_task the parent gets lost when 
the task completion is updated.

From what I could see, this stems from some confusion between 
$task->parent and $task->parent_id when loading/saving/merging the task.

The problem is probably in Nag_Driver_sql::_buildTree() but can also 
be seen in Nag_Task::merge() in the lines 562-565:

              } elseif ($key == 'parent') {
                 $key = 'parent_id';
             }
             $this->$key = $val;

parent contains the data from parent_task, but the substitution above 
becomes a problem, since both parent and parent_id are already present 
in the task being merged, which results in it being overwritten when 
the $key is actually 'parent_id' (which comes after $key == 'parent') 
in the order of the properties.

I felt that tasks being passed around like this requires a more solid 
understanding of the framework, so I haven't studied it much further 
than this.