Default bar numbers

[Patrick or Cynthia Karl]

Even though the default is for bar numbers to be printed at the beginning of 
every line of a piece except the first one, there are circumstances where a bar 
number is not  printed at the beginning of a non-initial line.  For example, 
the following snippet generates two lines, but the second line does not get a 
bar number:

\version "2.16.2"
\relative b' {
        \repeat unfold 5 { a b c d }
        a b c
       \break
        \repeat volta 2 {
                d
                \repeat unfold 5 { a b c d }
                a b c
        }
}

Is there a way to force a bar number at the beginning of line 2?  Is this a bug 
or a feature or a documentation problem?  Or have I just missed something?