If you go into the memberships and change the membership to individual, then remove one of the spouses as if the couple had divorced, then run an ETL refresh you will get null membershipprogramdimid in the dim_membership table.
Download and install the latest patch, which contains all fixes from previous patches. If you are running an older version, download and install the latest version and then the patch.
Steps to Duplicate
1. Locate a joint membership of two spouses. 2. Change the membership from joint to individual. 3. Remove one of the spouses, as if you are divorcing the couple. 4. Run an ETL refresh. 5. Note that the table results in null membershipprogramdimid in the dim_membershiptable