changing part combine mode breaks spanners

[Shevek]

I use partcombine a lot because I am working on a full orchestral score and I
need to switch between writing e.g. clarinets on a single staff and
clarinets on separate staves. Because of the demands of this notational
situation, I always use the manual partcombine commands, rather than the
automatic mode. Unfortunately, the manual partcombine commands only sort of
work. One particularly annoying limitation that I am running into is that
switching manual partcombine modes in the middle of a slur or other spanner
creates unpredictable behavior, such as hairpins going off to infinity.

In a tiny example:

\version "2.16.0"

obI = \relative c''' {
  \partcombineSoloI
  g1(~\p\< |
  \partcombineApart
  a1)\f |
}

obII = \relative c' {
  R1 |
  f4 e f g |
}

\new Staff = "1" \partcombine \obI \obII

The slur and the dynamics of the first measure are lost because of
\partcombineApart. Obviously, this example is musically ridiculous. This
sort of situation comes up quite often, though, when I need to have the
second voice enter in an additional staff while the first voice continues a
phrase.

Is there a workaround to this issue? Is this a bug (I don't recall seeing it
on the list of bugs for partcombine)? 



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