Unkown Element Tremolo

This is an odd situation so I'm not sure if anyone will be able to help, but here goes. I'm writing a program that simulates cancer growth and treatment in a musicxml song. It replicates and modifies the actual xml source. I have it working well in most cases, but I can't get several songs to convert from mxl to mp3. There are usually some warnings at this stage, but most songs can be converted in the end. In cases where it fails, the final error messages are something like this:

cannot add rest at tick 114960 track 16: element already present
ChordRest::add: unknown element Tremolo

The songs that run into this do have a Tremolo element and the therapy simulation replaces a lot of notes with rests, so I figured the error was from a Tremolo overlapping with a rest. I've tried removing the notation block that contains the Tremolo field (both in modified song parts and from the entire score), but it still runs into the same error. Is this represented in some other form in the score?

I know I'm trying to do something musicxml was never meant for, but if anyone could point me in the right direction it would be greatly appreciated.