In this case, the location rule for the taxonomy “chapter” was based on the slug “3-groupage_by_road”. My client later changed the slug (via the breadcrumb tool in Yoast SEO, but direct changing of the slug caused this too), but the ACF location rule did not change. So the fields weren’t available anymore.
Could this be changed to the term_id instead of the slug? Or are there complications that I am not aware of?