e. g. in the following code:

  <owl:DatatypeProperty rdf:ID="sessionLanguage">
    <rdfs:domain rdf:resource="#Session"/>
    <rdf:type rdf:resource="http://www.w3.org/2002/07/owl#FunctionalProperty"/>
    <rdfs:label>Session Language</rdfs:label>
    <rdfs:range rdf:resource="http://www.w3.org/2001/XMLSchema#language"/>
    <rdfs:comment rdf:datatype="http://www.w3.org/2001/XMLSchema#string"
    >The preferred language to be used during a session.</rdfs:comment>
  </owl:DatatypeProperty>

the line with the "<rdfs:range ... " is marked yellow with the warning

  Use of the property implies that object is of type
       http://www.w3.org/2002/07/owl#Class

But I believe it matches the OWL specification.  It must be a bug!