ugly slur in TabStaff if font is changed

[Federico Bruni]

If I change font (any font) in TabStaff, a slur in TabStaff gets ugly 
(only in 2.15.39 and latest git, not in 2.14.2).
It doesn't occur in any slur, just in some.

Find attached the png previews: the problem is in the last two notes.
Here's the minimal example:
## Snippet

global = {
  \key d \major
  \time 4/4
  \override TabStaff.TabNoteHead #'font-name = #"Nimbus Sans L Bold"
}


first =  \relative c {
  \global
  a8( b c) d~ d a e'( d) |

}


second =  \relative c {
  \global
  d,4 d d d |

}

\score {
\new StaffGroup <<
    \new Staff = "guitar" <<
      \context Voice = "first voice" { \clef "G_8" \voiceOne  \first }
      \context Voice = "second voice" { \clef "G_8" \voiceTwo  \second }
    >>
    \new TabStaff \with { stringTunings = #guitar-open-d-tuning } <<
      \context TabVoice = "tab first voice" { \clef "moderntab" 
\voiceOne \first }
      \context TabVoice = "tab second voice" { \clef "moderntab" 
\voiceTwo \second }
    >>
  >>
}
<<attachment: tab-slur-2.14.2.png>><<attachment: tab-slur-2.15.40.png>>_______________________________________________
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