\version "2.12.3"

ts = #(make-music 'TextScriptEvent 'text "t.s." 'direction UP )
% fb = \figuremode { <5>4 s4^\ts <3>2 } % Works
% fb = \figuremode { <5>4 s4-\ts <3>2 } % Works
fb = \figuremode { <5>4 s4\ts <3>2 } % Works
% fb = \figuremode { <5>4 s4_\ts <3>2 } % Does NOT work!!!
n = \relative c' { c4 d e f }

\score { <<
    \new Staff <<
      \new Voice << \n \fb >>
    >>
  >>
}