clef_8

[bb]

I have a problem with the 8 under the clef clef_8. That number 8 does
not disappear with    
\override Staff.Clef #'stencil = #ly:text-interface::print
I cannot decide if this is a bug?

Thanks for any help!

Regards

Example code:

\version "2.19.2"
   MTbass-five = \stringTuning < dis, g, b, dis g >
                       
roottoroot-one  = {
    \clef "bass_8"         
    \transpose g g, { g8 a b d' c' bes a g \bar "||" \break }
}

chordroottoroot-one =\chordmode { g2:7 c:7 }

#(set-global-staff-size 27)

\score {
  <<
    \new ChordNames \chordroottoroot-one
    \new Staff \transpose c c, \roottoroot-one
    \new TabStaff {
    \override Staff.Clef #'stencil = #ly:text-interface::print
    \override Staff.Clef #'text =
    \markup
    \whiteout
    \raise #1.0
    \bold
    \override #'(baseline-skip . 0.5)
    \center-column {
    \abs-fontsize #12 { g dis b, g, dis, }
    }
    \set TabStaff.stringTunings = #MTbass-five
    \roottoroot-one
    }
  >>
  \layout { }
    \midi { }
}