Strange, but we've seen it before too. The usual cure is to leave the rear washer out, if you're still using one.
I think I even ended up using a thinner washer under the nut as well, but can't remember.
Is yours running a thick washer under the nut and another between the bushing/sleeve and the mount? Or is the bushing right up against the mount?
I seem to remember one that was so extreme the guy had to source a thinner nut!
Good luck.
Paul