(bug?) irregular lyrics distance in 2.13.10

[Wilbert Berendsen]

Hi, is this known?

\version "2.13.10"
\new ChoirStaff <<
  \new Voice = "top" { c' d' e' f' }
  \new Lyrics \lyricsto "top" { p x x x  }
  \new Lyrics \lyricsto "top" { a x x x }
  \new Lyrics \lyricsto "top" { d x x x }
  \new Lyrics \lyricsto "top" { a x x x }
>>

yields attached PNG.

The lyrics vertical spacing is influenced by the precence or absence of 
extents in the letters.

best regards,
Wilbert Berendsen

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