Custom Taxonomy – boxes won’t check

I have a custom post type with custom taxonomy of years registered to it.

It was working fine but then the client made an update (unclear what) and now it has stopped working. If previous posts had a year checked, if you try to check a different year, all the checkboxes become cleared out.

Inspecting it, I can still the functionality still works, the input fields have checked=”checked” if you click on them, it’s just that the ::before with WordPress’s default blue checkmark doesn’t appear on input fields with check=”checked” like how it does with other categories

Not sure how to fix?