slurs problem with voices

[Robert Garrigos]

I have this music to engrave, note the slur:




I tried different approaches but cannot get that slur. This is my code, with 
two examples:

\version "2.25.14"
\language "english"

global = {
  \key ef \minor
  \time 3/4
}

first_example = \relative c' {
\clef treble
  \global
  <<
    {bf'4 \once \override Tie.staff-position = 5.1 cf2~ | <ef, gf cf>8 <ef gf 
cf>8 r8 <gf cf ef> r8 <ff bf df>}
    \\
    { d4 d2 |  \hideNotes (ef8)} % this slur doesn’t get printed
    \\
    {s4 \once \override NoteColumn.horizontal-shift = #1 f  gf8 af  | s2.}
  >>
}

second_example = \relative c' {
\clef treble
  \global
  <<
    {bf'4 \once \override Tie.staff-position = 5.1 cf2~ | <ef, gf cf>8 <ef gf 
cf>8 r8 <gf cf ef> r8 <ff bf df>}
    \\
    { d4 (d2 |  \hideNotes ef8)} % this slur does get printed
    \\
    {s4 \once \override NoteColumn.horizontal-shift = #1 f  gf8 af  | s2.}
  >>
}

\score {
  \new Staff \first_example
}

\score {
  \new Staff \second_example
}


first example prints no slur:



second example prints a slur where it is supposed to be, according the code, 
although it is not what I want:



Why is not the first example working? I tried with explicit voicing with the 
same results.

Thanks.

Robert