RelatedTermString plugin discards multiple values

Problem/Motivation

If you have a multi-value text field mapped using RelatedTermString to a taxonomy reference field, we have logic to inject new terms as needed and facilitate multiple values. (see https://www.drupal.org/project/salesforce/issues/3165707). But currently, if you pull multiple values from SF the code discards all but the last one in the list, which will then get pushed back to SF and cause data loss on the next update in Drupal.