Have you tried with a different lens?
I don't get the same result as you are getting, but I am getting magenta streaks on one of my shutters on 'B'. It was CLA'd at the beginning of the year but was not releasing immediately.
It has been adjusted and doesn't appear to stick now, but it still doesn't record an image without any magenta. It's okay on other shutter speeds.
I'm not sure if this might give you any further ideas.
Ray