Comment #1 on issue 2802 by [email protected]: NoteColumn interleaves with KeySignature of other Staff
http://code.google.com/p/lilypond/issues/detail?id=2802
It has to do with how LilyPond deals with horizontal skylines. The MultiMeasureRest is a spanner and thus doesn't get horizontal. It may be high time to create MultiMeausreRestStub item that reserves space for these spanners (takers?).
One way to fix it is to do \override Staff . KeySignature #'transparent = ##t \key ces \major
