Because it's not found in the "pendingTypes" list. The reason why it's not there (yet) is that it has not been added yet (when the [orc:patient] is parsed)

So from what I can tell, [orc:patient] is parsed first (and added to the list) and then the validation of the residual property of type [orc:patientcase] occurs (but that type has not been loaded yet).

Based on the javadoc from findTypeInMapOrList, _null_ seems to be a valid return, meaning "not found"