Yes, that is all correct. I changed the column to a varchar in MySQL database schema and all was good.
But still wondering if a TypeError would not have been more appropriate, given:
plone.app.vocabularies: 4.2.1 → 4.2.2
Bug fixes:
Change vocabulary tokens to use base64.urlsafe_b64encode().
No newlines and safe to use as an xml attribute.
See community post <https://community.plone.org/t/tags-subject-field-mangling-long-terms/13067>_.
[flipmcf] (#64)