#5026 \crossStaff mangles all grace note flags

[Simon Albrecht]

Reported by Harm:

\version "2.19.53"

A = {
  c'8 \grace c'8 c'8
}

B = {
  \voiceOne
  \autoBeamOff
  \crossStaff \repeat unfold 2 { \grace { c'8 } c'8 }
}

\new PianoStaff \with {
    \consists #Span_stem_engraver
} <<
  \new Staff \A
  \new Staff \B
>>

Same behaviour for 2.18.2.