Are you sure you are following the 67 at the moment of actual exposure? MLU comes first (initiated by you), then you shortly after, triggering the shutter and, by dint of that, stopping down the aperture blades. It is not in the realms of possibility for the process to be reversed or for MLU to emulate the shutter: it has no connection to the shutter; it's only connection is to the mirror with electrical connection to two solenoids to bring it up and magnets to throw it down. But...

...if the prism/lens reattachment process is not followed, unpredictable results can occur because the aperture coupling is not engaged, so there is no coordination between the lens aperture and the camera coupling. Remove the prism. Remove the lens (this resets the position of the aperture coupling to null. Then replace the prism. And replace the lens, in that order (this will link the aperture tang and coupling chain correctly). Load a dummy/exposed roll of film and go through the process carefully again.