Tablature, unwanted stroke of tied note

[Francisco Vila]

Hello all,

in this example

upper = \relative c' {
  r2 <b d f>2~
  <b d f> <b b e>2~
  <b b e> r2
}

\score {
  \new StaffGroup <<
    \new Staff { \clef "G_8" \upper }
    \new TabStaff { \upper }
  >>
}

the second chord has the same note twice, and that seems to confuse
the tablature engraver making it to show a figure as if one of the
notes had to be stroke again. In the standard staff we can see three
ties on both chords. Image attached.

Do you know why, and how to avoid that? Thank you!

-- 
Francisco Vila. Badajoz (Spain)
www.paconet.org , www.csmbadajoz.com

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