Light leak from WLF on P67 long exposures ?

I am trying to figure out what hapenned on these long exposure shots (around 30 seconds). I was using the WLF, the 45mm lens (and a tripod obviouly). I think I left the WLF opened after focusing. Is this a known problem? Can the WLF cause light streaks on P67 long exposures ?
Thanks!

I'm assuming these were all shot with the MLU switch on? Take the lens off and cock the shutter and then trip the mirror lockup. Now look to see if the mirror is all the way up with no gaps. You can lightly touch the mirro in the up position to see if there is any play. Now, with the mirror still locked up, use a cable release and set the shutter to "B" bulb. When you trip the shutter release watch the mirror to make sure it stays in the total lockup position.

Thanks for the useful reply! I just performed the test that you suggested. There is no play (maybe half a mm movement when pushing with a q-tip, I don't think it qualifies...) and the mirror stays well put when releasing the shutter.
Again, is it a possibility that the problem comes from the waist level finder left "open". I know I left it open (as in the attached picture) while exposing. It didn't cross my mind it could cause a problem. Now I remember several models of 35mm reflex cameras do have a viewfinder blind for long exposures. That gave me the idea that maybe the open WLF could be the problem. Could it make sense ? Or do you use it this way without any trouble ?
Of course I can test it next time taking the same shot with the WLF opened and then closed.

WLF itself cannot cause light leaks. It's natural to bleed light onto the focusing screen by design.

Hassasin said:

WLF itself cannot cause light leaks. It's natural to bleed light onto the focusing screen by design.

Click to expand...

It occurs to me that a different finder might block light from a problem area in the body, while the WLF lets the light through to that problem area.
In that case, using the WLF merely reveals the issue.