A new problem with my Pentax 67. Putting the speed at 1 second caused a strange shutter problem today, one that gave me no choice but to waste half a roll trying to figure out what was going on. When I put the shutter dial at 1 second and pressed the MLU switch, the aperture blades stopped down to f/22 (correctly), for about a second (judging by ear), then the mirror went up and the blades opened up again. Then, upon pressing the shutter, the the mirror went down immediately, leading me to believe the MLU switch made the exposure. How can this be!?

I can't believe how mischievous the 67 is. After unloading the wasted roll when I got home, I tested the camera in dry mode at 1 second again with MLU engaged. Worked perfectly!
Why does the shutter suddenly take on a life of its own when film is loaded?