Sounds like it has a problem.
When you hit the pre-release, it should close the shutter in the lens then the mirror goes up and the rear curtains open. When you press the shutter release the shutter should open and close, then on releasing the button the rear curtains will close.

Should work the same way whether or not the lens is in DOF preview mode.

Based on what you describe with your camera, I'd expect a fogged frame if you pre-release with film in a back.