Second staff beginning with an acciaccatura defeats "\override Staff.TimeSignature #'stencil = ##f" in the first staff

[Štěpán Němec]

% here, the first staff gets time signature, but shouldn't

\version "2.14.2"

\new PianoStaff <<
  \new Staff {
    \override Staff.TimeSignature #'stencil = ##f
    c
  }
  \new Staff {
    \override Staff.TimeSignature #'stencil = ##f
    \acciaccatura c c
  }
>>

============

% this works as expected, no time signature

\version "2.14.2"

{
  \override Staff.TimeSignature #'stencil = ##f
  \acciaccatura c c
}

============

% also this works as expected, the problem only occurs with
% acciaccatura at the very beginning

\version "2.14.2"

\new PianoStaff <<
  \new Staff {
    \override Staff.TimeSignature #'stencil = ##f
    c
  }
  \new Staff {
    \override Staff.TimeSignature #'stencil = ##f
    c \acciaccatura c c
  }
>>

-- 
Štěpán