I am trying to update via workflow the tax item on purchase orders or vendor bills based on location. I tested by billing an existing purchase order and the workflow logs show that it executed but the field is missing. Am I missing something or is the tax item on the line level something that cannot be overridden?

Is the field present on the form?

Yes, field is present on the form

Are you hitting the right tax field? The form's customization will show many tax fields but they do not necessarily function which I think depends upon Nexus. for instance, I can see PST on customisation but that only appears on Canadian documents.

Yeah, after further research it seems like tax item/tax code cannot be changed via workflow.