Facets hierarchical taxonomy terms not working I use the Facet module version 8.x-1.x-dev. I have a list of taxonomy terms with parents and childs. 

The taxonomy term facet is an Entity reference field. I can't get the hierarchical structure working in the frontend.
I fixed it by re-added the field from admin/config/search/search-api/index/default_index/fields and take the ID from the index instead of name. Then in the Facet settings use Transform entity ID to label. Then the hierarchical structure from the taxonomy terms works!