potential stem-direction bug

[Graham Percival]

Should the b4. stem be going up?  LilyPond places it going down.  I 
don't know
enough about engraving to say whether this is actually a bug or not.


\version "2.3.16"
\bookpaper{ raggedright = ##t }
{ \relative c''{
  \time 5/8
  b4.( ~ b8[ a)]
}
}