\version "2.18.2"

 \addInstrumentDefinition #"timpani"
  #`((shortInstrumentName . "Tmp.")
     (clefGlyph . "clefs.F")
     (middleCPosition . 6)
     (clefPosition . 2)
     (instrumentCueName . ,(make-bold-markup "timp."))
     (midiInstrument . "timpani"))
  
\header {
  title = "Bugged-out instrument switch"
  subtitle = "Seems related to using a \common voice"
  subsubtitle = "Which are a good way to store time & key signatures"
  composer = "CJM"
}

common = {
  \key gis \minor
  \time 11/16
  s1*11/16 \bar "||"
}

timp = \relative c' {
  r4
  \instrumentSwitch "timpani" 
  b,8 b16 b8 b
}

\new Score { <<
  \new Staff { << \common \timp >> }
  \new Staff { \timp }
>>}