Bug in spacing of lyrics in polyphonic context

[P Scott Horne]

In the old days of version 1.6.8, we wrote polyphony this way:

        <{c d e}\\{e f g}>

Now the notation is

        <<{c d e}\\{e f g}>>

There seems to be a bug in the engraving of lyrics set against a staff that
breaks into polyphony with the new notation of double chevrons.  In version
1.6.8, the horizontal spacing was correct; in version 2.2.5, it is not.
Syllables appear much too far to the right of the notes against which they
should be aligned.

This is how I set up the score:

        \score {
         <<
          \addlyrics
          \context Staff = melody
          {
             [...]
          }
             \lyrics \context Lyrics <<
               \context Lyrics = "text1" {
                 \set stanza = "1."
                 \textA }
               \context Lyrics = "text2" {
                 \set stanza = "2."
                 \textB }
               >>
          [other staves, &c]

If necessary, I'll prepare a simple example that can also serve as a
regression test.

Scott Horne