I should have clarified that I'm not looking for a workaround for this. I probably should have submitted this as a bug report instead of posting it to the forum - sorry!
I could edit the MusicXML file by hand and remove the rests, manually adding the time to the preceding note. That would be an interesting experiment, to see if it helped Sinsy generate better output. I'll probably do that if I get some spare time.
But my current project is a home-brew voice synthesis program, and I figured it would be neat if it could read BiaB's Music XML files directly.
For that, I've come up with a workaround where a rest following a note is automatically assigned to the prior note's duration if the duration is below a certain threshold. It's not optimal, but it works.
I'm just noting that understanding Sinsy's output is already difficult, and the addition of rests isn't likely to make the behavior any better. Plus, fixing this would make
my life a bit easier.