midi export deletes notes under specific circumstances (chords, ties, voices)

[OttavioCresos]

9.9.1

Mac OS Sequoia 15.6.1

Problem summary

Under specific circumstances (e.g. ties + chords, 2 piano voices) parsing a musicxml file with the converter, and then immediately writing a MIDI file from it, will result in notes missing and moved/overlapping.

We suspect that this could be related to the stripTies method.

To illustrate and reproduce the issue, I created two minimal examples of a single bar. With several bars, more related issues appear, but for now:
example_doubleVoice_2notes.musicxml:

example_doubleVoice_3notes.musicxml

I'm also providing the resulting MIDI files.

Steps to reproduce

import music21
converter = music21.converter
score = converter.parse('example_doubleVoice_2notes.musicxml')
midi_path = score.write('midi', fp='example_doubleVoice_2notes.midi')

Expected vs. actual behaviour

for the first example, this what the output should be (midi exported from MuseScore) as seen in sonic Visualiser:

this is the music21 output, with 4 midi events instead of 2:

for the second example, this what the output should be (midi exported from MuseScore) as seen in sonic Visualiser:

this is the music21 output, with 4 midi events instead of 3, note that the second (top) voice has been deleted:

I'd be happy to produce more examples if this can help to fix this issue, thank you so much!

issue_report_example_files.zip