Ok, it did work for a while and now it seems not to work again. Can you please check the attached file?
What I did is I copied one MOP from another part with different stock. And it seems to rememer it somehow.
Edit: Now I see, I need to have the part selected by the cursor, I had selected one of the MOPs inside the Part. If the cursor is on the part, it selects the correct stock size. "Active" part has no influence on this (I had it under cursor before, that is what confused me a bit).