As it is not possible to transport changes on decision workflows via a change label, instead, transport the workflow as a "container".
That means, objects which are assigned to the container object in the target database, but are not contained in the Transport file, will be removed in the target.
For that, use the Transport mode "selected objects and dependencies".
Select the PWODecisionSubMethod (to transport only the workflow) or the the PWODecisionMethod (to transport the complete approval policy).
The system automatically determines the dependent objects.
Important: Please deactivate the relation to PWODecisionRule. Otherwise the import may try to remove DecisionRules in the target (because of the described behavior).
Then, import the new Transport again. That should repair the workflow in the target database.
Defect ID 29985 has been logged to resolve this in a future release of Identity Manager.