When a fact type is objectified and the model is generated with OntoUML stereotypes, the links between the resulting class and the object types playing its roles are all labelled as material relations.

Those links are mediations: they connect the objectified relationship to the participants it depends on. A material relation is something else, namely the derived relationship between the participants themselves.

The effect is that an objectified fact type is shown as a cluster of material relations, which hides the distinction the stereotypes are meant to make visible.

Expected: links from an objectified fact type to its participants are labelled mediation.