Problems with polyphonic playback

I discovered an issue with measures that begin splitting into polyphonic voices.

If two voices start with notes of the same pitch only one voice will be heard. If the starting notes are different pitches then both voices are heard thereafter.

Example begin a polyphonic measure with voice 1 and 2 starting on middle C. Voice 1 is a moving melody line while voice 2 sustains the middle C (e.g. a whole note). Only the melody is heard. Move either pitch to something else and both voices are heard thereafter. Put the notes back to the same pitch and the sustained note disappears. Interestingly the moving line is always heard regardless of which voice it is assigned.

See the measures 15 and 39 on the score snippet I attached for an example.