Custom Post Types using wrong template (index) instead of archive-{type}.php – previously worked as expected

The issue was, in fact, the new taxonomy using “year” clashes with the query.

WordPress was accepting year=about-us in the query chain for instance, as year was a taxonomy, before the expected page=about-us – preventing it from appearing as a page and using the index.php template as there is no “year” template.

I changed year to a more custom slug edition-year in functions.php and flushed the permalinks, which resolved the issue.