Successive crescendo/decrescendo fades to silence

[Javier Ruiz-Alma]

This tiny example with swells of crescendo-decrescendos compiles without warning, typesets as expected, but the midi unexpectedly fades to silence.  Terminating the crescendos with \! before adding the decrescendo fixes the issue, but NR shows this as valid syntax.  Bug?

\version "2.18.2"

\score {

   \new Staff {

     \new Voice {

       \relative d' {

         \time 3/4

         r8 d\mp e f f g

         \repeat unfold 4 {

          r8 d\< e f f\> g\!

         }

       }

     }

   }

\layout { }

\midi { }

}