invisibly short ties in multiple voices

[Jim Cline]

I am frequently getting ties that are so short as to be invisible.  This 
was discussed in a previous thread, and I was happily using the solution 
given by 
https://lists.gnu.org/archive/html/lilypond-user/2015-02/msg00069.html, 
which is to insert \once \override Score.NoteColumn.X-offset = 1.5 between 
the tied notes, forcing the tie mark to become longer.  However I find it 
is not working when I have multiple voices, as in the following example. 
It works as expected in the lower staff, but not the upper staff.  Somehow 
the presence of the second voice seems to be canceling out the added 
horizontal space.  If I replace the music in the second voice by a 
complete measure of silence (s2.) then the tie gets lengthened as desired.

(Incidentally, \stemDown is being ignored in the lower staff, I don't 
understand why).

\version "2.20.0"

bspace = {\once \override Score.NoteColumn.X-offset = 1.5}

upper = \relative c'' {
  \clef treble
  <<{\stemDown \grace{d16_~}\bspace d4 e fis}\\{\stemUp  f16 e f8 s2}>>
}

lower =
  \relative c {
   \time 3/4
  \clef bass
 \stemDown\grace{ d'16_~}\bspace d4 e fis
}

\score {
  \new PianoStaff <<
    \new Staff = "upper" \upper
    \new Staff = "lower" \lower
  >>
}