I have mainly C lenses and only one KL. Don't forget I am using a P45.
I did a quick test this morning and there is a difference between M and X, I get a magenta cast with X, I think this is because the sync connection is fired before the shutter is fully open.
I am at a wedding now and do not have the DB with me.
I can only assume there is a communication error between the sync and the back but if you do as Kumar suggests then we may have more idea.
As I say I had a lens for a 5x4 camera which I use on my dMini, this produces a purple cast when on the B setting. It was sticking so I had it CLA'd, when it came back I had a purple cast, I returned it and it was redone. I still get the purple cast but not as bad. I think it is a problem with the shutter in the lens.
I do think you would be extremely unlucky to have a problem with three lenses (I have 7 for the Mamiya, some of which are duplicate and I have not had a problem).
I do not know how the camera wakes up the H20, I think on the P45 it is a bit mechanical.
I will wait till you have tried the Sinar lens, before your try and tape it onto your Mamiya can I suggest that you use a Mamiya lens on the camera with the back, and connect the sync cable to the back and the Sinar.
My thinking is that then you press the shutter, this will wake up the back and open the lens BUT NOT expose the back. The actuate the Sinar lens, this will then expose the back and end the exposure (set the Sinar to an fixed exposure). You can then press the lever on the Mamiya.
I would suggest that if this works it is the Mamiya lens that is causing the problem, if its the same then I would suggest its the initial wake up.
If its the same I would do what Kumar suggests and mount the Sinar on the camera but it will have to be light tight as I have found any leakage may cause the problem - and just having had a lightbulb moment, maybe this is the problem - you have a light leak?
I may not be able to check for a couple of days, but will check then.