rests colliding with beams when voice changes staves

[jeff covey]

take a look at measure 17 here:

http://jeffcovey.net/music/scores/bach/inventions/12-tmp/

when the upper voice moves into the lower staff, the eight rest in the
lower voice jumps up and engraves itself over the beams of the upper
voice.  it looks like it's trying to get out of the way and jumping in
the wrong direction.

i was able to correct it with

\property Thread.Rest \override #'staff-position = #-11

here:

http://jeffcovey.net/music/scores/bach/inventions/12/

, but that's kludgey.  is there something else i should be doing, or
is it just a bug in lily?

thanks,

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