Partcombine without slurs or ties?

[Richard Shann]

In the following example:
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\version "2.18.0"
MI = { \time 3/4
         \override Voice.Slur.stencil = ##f                   
         \override Voice.Tie.stencil = ##f 
          %comment out the next bar to suppress the slur and tie                
          
          c'8~ c' c' c' c' c' 
          c'8( a'8) c'' c''8 c'' c''8
     }
MII = { \time 3/4 
         \override Voice.Slur.stencil = ##f                   
         \override Voice.Tie.stencil = ##f 
          %comment out the next bar to suppress the slur and tie                
          
         c'8~ c' c' c' c' c'  
         c'8~ c' c' c' c' c' 
      }
\score {
  \new Staff << 
               \partcombine  
               \MI  
               \MII
             >>
       }
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I find the slurs and ties are drawn despite no stencil. Stranger still,
the *are* suppressed if I comment out the first bars.

Any ideas?

Richard Shann