I’ve managed to get around this issue, but let me explain what I did and see if we can hunt this down. WP Engine insisted it was too hard to figure out.
I created a new category and updated the code to filter by that new category.
I ran an update on the term_relationships table to change everything associated with the old category to the new one.
On events series, I had to de-select the category, save, and then re-save with the new category.
It now…works. Any thoughts?