Hi Steven,
I've created a DDL export from one of these problematic xmi files. The primary keys did not contain any nullable field. So I don't think that this is the problem.
I did notice that not all the tables contain a primary key field. However the database did and the import functionality did not copy them. This database schema was generated from a Sybase database.
I'll try to create a new import using teiid designer 8.5 later on.
If you get a chance, open an issue with Teiid Designer using your model project set and then we can take it from there as to what is happening.