Since #11425, it appears that smart class parameter overrides based on FQDNs, and presumably host groups, are not being deleted when the host is deleted. This causes the lookup key to later fail validation when changed.

It looks like the dependent destroy was removed at https://github.com/theforeman/foreman/commit/9dd005342b8424bdf5453b9b0e0a27989b7f3079#diff-4d26af5acbc256436626e7dbe476e3deL33