Thanks for the suggestion. I changed the BO, but still got the same result. After some trial and error, I found a solution. The customer tableset doesn’t directly include CustAttr. Instead the entries in the table are in Customer.AttrCodeList. Apparently the Update method thinks that anything that is in this field should be added to erp.CustAttr. So, my solution is to set that field to “” when I update dsCustomer.
Fortunately, the BO doesn’t appear to think that is should remove entries in erpCustAttr when if the AttrCodeList is empty. 
Now the data directive works as expected.