Strange interaction between \time and \grace

[phil . auclair]

Hello,

Something strange seems to happen when I specify the time signature in several 
staves, then write a grace note just after. Two time signatures are printed, 
with the grace note between them.

In the following snippet, if I don't specify the time signature in the Tenor 
staff, or if I suppress the Tenor staff, the result is correct:

\version "2.13.0"
\include "italiano.ly"

SopranoMusic = \relative do'' {
        \time 4/4
        \grace do8 sol4 la si do
        \time 4/4
        \acciaccatura do8 sol4 la si do
        \time 4/4
        \acciaccatura do8 sol4 la si do
}

TenorMusic = \relative do'' {
        \time 4/4
        do re mi fa
%       \time 4/4
        do re mi fa
        \time 4/4
        do re mi fa
}

\score {
   <<
        \new Staff = "Soprano" { \SopranoMusic }
        \new Staff = "Tenor" { \TenorMusic }

   >>
  
  \layout {
  }
}