that's expected. OWM manages the foreign keys internally when you're having version enabled tables, that's why you don't see them anymore.
please see chapter 1.9 ("Constraint support with Workspace Manager") in the documentation for more information.
when you open a DDL session on a version enabled table you can see the foreign keys on the temporary _LTS table. that's also the only way later on to add/remove foreign key constraints.
apart from that you can use the views ALL_WM_RIC_INFO or USER_WM_RIC_INFO to view foreign key metadata.