Its mentioned (in considerations, last point )that
If a subcase update attempts to update its parent, the system briefly locks the parent to effect the update. If the parent is locked, the updates are queued until a lock can be obtained.
In this case, since the current operator has lock on both child and parent cases, the expectation is that the update happens like how it happened for default locking case. Can you provide work object XML for parent case and child cases for validation?