By design, the behaviour of the navigation properties with IsManaged set to True, in scenarios where a child object is deleted from the collection of the parent object, is to remove the relationship between the two objects. In other words, Telerik Data Access (previously known as Telerik OpenAccess ORM) will keep the child record in the database but will generate a statement that attempts to set the foreign key to NULL.
A solution in this situation would be to pass the collection to the Delete method of the context. For example:
dbContext.Delete(User.Investments);
dbContext.SaveChanges();
This will produce the necessary DELETE statement. More details about the management of navigation properties with Telerik Data Access is available in this documentation article.
I hope you find this feasible. I am looking forward to your feedback.