D6->D8 "This entity (taxonomy_term: XX) cannot be referenced."

After upgrading a Drupal6 site to Drupal 8.5.3, while a taxonomy that existed for blog articles (“My Tags”) is displayed correctly in the upgraded site, and listing the terms of the “My Tags” Vocabulary shows all the correct terms from D6, and clicking on each of the terms shows the articles which were tagged with that term (for example taxonomy term “Politics” 20, http://site.com/taxonomy/term/20), attempting to edit an article then fails to save with the error  “This entity (taxonomy_term: 20) cannot be referenced.” The previously defined term is shown in the field as “Politics (20)” however, attempting to enter it, or any of the other terms are not able to be found when searching for existing terms.  Entering the same text “Politics” into the taxonomy field then proceeds to create a new term, listed as “Politics (45)”. I have then ended up with two identical taxonomy terms, but differing in taxonomy id. However, while this new term is created, it is not shown in any of the taxonomies that have been previously created. I literally can only reference it via site.com/taxonomy/term/45.

I have no idea where to begin to try and debug the issue, and this error prevents any updates to any content on the site, or indeed any new content, while a taxonomy term has been set as mandatory for each article. I’m not sure, but I think this is related to the bug referenced https://www.drupal.org/project/drupal/issues/2924783 ? Any pointers as to what I should even try to diagnose, in terms of SQL tables, or other diagnostics?

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/21/feed