\popContextProperty does not restore unset property

In this example, \popContextProperty appears not to restore the property 
to the state it was in (unset) before \pushContextProperty.  This is a 
bug, right?

\new Staff <<
   {
     \time 2/4
     \set Score.autoBeaming = ##f
     \skip 8*6
     \set Score.autoBeaming = ##t
   }
   \new Voice \with \voiceOne {
     \repeat unfold 12 c''8 % uses Score.autoBeaming
   }
   \new Voice \with \voiceTwo {
     \repeat unfold 4 a'8 % uses Score.autoBeaming (#f)
     \pushContextProperty Voice.autoBeaming ##t
     \repeat unfold 4 a'8 % uses Voice.autoBeaming (#t)
     \popContextProperty Voice.autoBeaming
     \repeat unfold 4 a'8 % should resume using Score.autoBeaming (#t)
   }
 >>

-- 
Dan