When trying to bulk edit parent for Defects and User Stories, the Parent/Portfolio Item field is not available to be selected
This is an architectural issue. Defects and Stories get parented in unique ways, in spite of the way that the UI makes it look when you're viewing the details page.
Bulk editing will typically only show the fields that the two work item types have in common, and since those parent fields are technically different on the back-end, they get removed from the dropdown when a user selects both work item types.
Bulk parenting will have to be done in two passes; one for stories and another for defects.
This is unlikely to be resolved as an enhancement due to the complexity of the issue and the risk involved in changing the behavior.