OntoUML classes with the stereotype «datatype» that have no attributes are interpreted as "primitive datatypes" by the plugin, which deserializes them as instances of IDataType. However, instances of IDataType cannot have relationships connected to it, for example, generalizations.
I think we can overcome this issue in three different ways in order to make a solution that is transparent to a Visual Paradigm user:
add some tagged value on the serialization to identify datatypes serialized as classes (will not be enough if the model was serialized in some other tool)
fix a pre-defined list of primitive OntoUML datatypes (possibly not trivial)
check whether the «datatype» class has relationships or appears in some diagram, deserializing it as a class if so (arguably the most elegant option, but requires changes in the Java "metamodel" for OntoUML)
OntoUML classes with the stereotype «datatype» that have no attributes are interpreted as "primitive datatypes" by the plugin, which deserializes them as instances of IDataType. However, instances of IDataType cannot have relationships connected to it, for example, generalizations.
I think we can overcome this issue in three different ways in order to make a solution that is transparent to a Visual Paradigm user: