The terrain model is defined by the features, but the features have value by themselves, so they are not deleted. For example, you may have an EOP as one of the features. This has value for a variety of purposes.
Note, that when importing a DTM file, you have an option of importing features or not. If this is true, then the behavior you descirbe is expected. If you do not set the option to import features, then the terrain model will be identical but the breaklines (features) are embedded with the terrain model eleent and all delete at once.