Wrong MIDI-output when notes of the same pitch appear in different voices

[Ansgar]

%In the first bar the MIDI-output is as desired.

%In the second bar, the first a'4 is changed to a'8 a'8. 
%The created PDF is fine.
%However, the MIDI-output cuts the following Note, a'2, down to a quater.
%I.e. it ends, where the a'2 from voiceTwo ends.

\score {
        \new Staff {

                
                <<      \new Voice{\voiceOne a'4 a'2 a'4}
                        \new Voice {\voiceTwo a'2  f' }
                >>


                <<      \new Voice{\voiceOne a'8 a'8 a'2 a'4}
                        \new Voice {\voiceTwo a'2  f' }
                >>

        }       
        \layout {}
        \midi {}
}

\version "2.19.42"