volta repeat and grace notes

[Mike]

I have a score with a grace note in a volta alternative. I get the
following error:
warning: already have a volta spanner, ending that one prematurely

and the result shows an extra little repeat bracket with just the
grace note it.  With on voice, or two voices on one staff everything
is fine - the wrongness only appears on a staff system.

Both 2.14 and 2.15 give the same result.

Looks like a bug to me, or at least very unintuitive behaviour. Is
it known, with or without a workaround?

Thanks,

Mike

VoiceOne =  \relative e'' {
    \repeat volta 2 {
        \clef "treble" 
    a4 a a a
    } % 8
    \alternative {
        {\grace b a a a a} 
        {c c c c}
            }
    }


VoiceTwo =  \relative a, {
    \repeat volta 2 {
        \clef "bass" 
        c4 c c c
         }
        \alternative {
                {e e e e}
                {d d d d} 
                }
      
        }
\score { <<
        \new Staff  <<
            \context Staff << context Voice {\VoiceOne } >>
            >>
        \new Staff <<
            \context Staff << \context Voice { \VoiceTwo } >>            
               >>  
     >>}