Auto-increment on partial primary key with Entity Framework Core

Well those Data Annotations should do the trick, maybe is something related with the PostgreSQL Provider.

From EF Core documentation:

Depending on the database provider being used, values may be generated
client side by EF or in the database. If the value is generated by the
database, then EF may assign a temporary value when you add the entity
to the context. This temporary value will then be replaced by the
database generated value during SaveChanges.

You could also try with this Fluent Api configuration:

modelBuilder.Entity<Foo>()
            .Property(f => f.Id)
            .ValueGeneratedOnAdd();

But as I said earlier, I think this is something related with the DB provider. Try to add a new row to your DB and check later if was generated a value to the Id column.