Repeating predefined stanza labels

[Matthew Collett]

The following works as expected (2.14.1):
\score {
  \new Staff <<\repeat unfold 4 { g' g' g' g' \break } >>
  \addlyrics { 
    \set stanza = "1." This is verse one;
    \set stanza = "1." This is line two.
    \set stanza = "2." This is verse two;
    \set stanza = "2." This is the end.
  }
}

Each line has the stanza number shown.  

The result is different if the labels are predefined:
one = \set stanza = "1."
two = \set stanza = "2."
\score {
  \new Staff << \repeat unfold 4 { g' g' g' g' \break } >>
  \addlyrics { 
    \one This is verse one;
    \one This is line two.
    \two This is verse two;
    \two This is the end.
  }
}

Now the stanza numbers are shown only when they change.

Why is the behaviour different in the two cases, and what incantation  will 
make the predefined labels be shown each time?

Best wishes,
Matthew