Multiple taxonomy terms not importing to field from CSV

I have a typical situation where I have several taxonomy terms, two taxonomies. I would like to make multiple entries from a CSV file to associate with that field (example: node row 1 has field_term_a with term ID 1, 2, 3 in it and field_term_b with term ID 5, 6, 7). field_term_a and field_term_b are both set as permitting multiple references.

I can consistently get a *single* term on the field_term_a (eg the first term ID attaches correctly).

I tried formatting the cells by both the text of the term and the ID of the term. Also made all terms inside Drupal before importing.
1, 2, 4, -or more precisely they had quotation marks on Excel export like: "1, 7, 21, ","24, 13, 15, 16, 25, 29, 28, "
1 2 4
termname_a, termname_b, termname_c

With the first two the mapping is set reference by term id, and the latter with the reference by name.

Using the ID codes without commas gives me only the first term in the field. I couldn't find a spot in the code that was exploding them on the comma-space character pair so I'm not sure that is the right format.

I looked around at EntityProcessorBase.php, TermProcessor.php and EntityReference.php and didn't catch any good clues about this.

Workaround

Probably going to use views_bulk_operations and views_bulk_edit to insert terms manually, it should let me add individual terms to multiple nodes 'additively' so to speak.