OK... super annoying... if you set the soft_delete flag in your schema for a Propel project, and that table is related to others via a foreign key... those related rows won't be deleted (or soft deleted) if the parent row is deleted.
Solution? Well, override the delete method and implement the cascade yourself in software... annoying...
Perhaps this is one of the reason's that Doctrine is taking over... it automatically implements this solution for you.
Here are some references: