\! required for consecutive crescendos in \partcombine

[gnomino]

> I'm not top-posting.

Hi,

I'm using LilyPond version 2.11.57. According to the documentation, \! is not
required to terminate a crescendo/diminuendo that is terminated by another
crescendo/diminuendo. However, in the following snippet, the \partcombine seems
to discard the second voice unless the first crescendo is explicitly terminated
with a \!. Is this a bug or the intended behaviour?

\version "2.11.57"
\paper{ ragged-right=##t }
%% second voice is discarded
\partcombine { c''1\p c''1\< c''1\<   c''1\f } { R1 c'1\p\< c'1\<   c'1\f }
%% works as expected
\partcombine { c''1\p c''1\< c''1\!\< c''1\f } { R1 c'1\p\< c'1\!\< c'1\f }

Thanks in advance.