Merging noteheads with \mergeDifferentlyHeadedOn

[Nick Payne]

In the following, why is it that the first C# in the top and middle
voices don't merge unless I also use \once \override NoteColumn
#'ignore-collision = ##t before that note in either one of those voices.
The subsequent notes in those voices do merge successfully without
needing this override.

\version "2.17.5"

\new Staff <<
  \time 3/4
  \mergeDifferentlyHeadedOn
  \mergeDifferentlyDottedOn
  \new Voice = "1" { \voiceOne \relative c''' { cis4 b cis } }
  \new Voice = "2" { \voiceTwo \relative c' { a2. } }
  \new Voice = "4" { \voiceFour \relative c''' { cis8 e, b' e, cis' e, } }
>>