When POSTing to /api/v2/hosts and using host[lookup_values_attributes] hash, parameter host[name] is directly used for smart variable matcher (fqdn=name). When host[name] is just hostname and not FQDN the resulting matcher is incorrect.

As workaround it is possible to set host[name] to FQDN, but web interface can use just hostname, so API should handle it in the same way. Btw. host[lookup_values_attributes] is missing in API documentation.