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!