Stem direction wrong for white mensural music?

[Phil Holmes]

If I compile the following code (the start of Dufay's Ave Regina a3) with 
default settings, the stems follow modern style - some up, some down.  For 
ancient music (I'm not sure when this ended, but it certainly applies around 
the period up to 1500) the stems were always up for normal notes; - 
downstems showed other features - e.g. chromatic alteration on German 
tablatures.

It's trivial to fix with a \stemUp, but I'd suggest this is a bug.  Does 
anyone else have a view, before I add it to the tracker?

\paper {
 ragged-right = ##t
}

\score {
 <<
   \new MensuralVoice = "Cantus" {
     \override Score.BarNumber #'transparent = ##t {
       \clef "mensural-c1"
       \time 3/2
       \[ a' \breve \melisma b' \] a'1 \melismaEnd gis' \breve a' g'1 a' 
b'2 \melisma a' g' f' \melismaEnd e' \melisma f' d' c'1 b2 \melismaEnd d' 
\breve
     }
   }
   \new Lyrics \lyricsto "Cantus" {
     A -- ve, Re -- gi -- na cae -- lo -- rum
   }
 >>
}

--
Phil Holmes