Go to Topology and define a USER mapping for a specific resource
Populate the field "Schema to hold values for identifiers generated upon Create by the external Identity Store" in the second step of the user provision wizard. Compile the mapping in order to make it valid and save.
Open mapping again and remove value of "Schema to hold values for identifiers generated upon Create by the external Identity Store". Then save
Open again the mapping in edit and you'll see that the value is still there.