Misplaced tie/slur in middle B when stem is upside

[Jordi Nadal]

When a B whole note is tied or slurred with a B with the stem pointing upwards,
the tie/slur is placed above notes instead of below.

I hope this code below will be more useful than my poor explanation.

\relative c'' {

  b1~^"Correct" | b4 c r2
  b1~ | b8 c r4 r2
  b1( | b4) c r2
  b1( | b4 c) r2
  \break

  a1~^"Correct" | a4 g r2
  a1~ | a8 g r4 r2
  a1( | a4) g r2
  a1( | a4 g) r2
  \break

  b1~^"Incorrect" | b4 a r2
  b1~ | b8 a r4 r2
  b1( | b4) a r2
  b1( | b4 a) r2

}