TextureAtlasExporter incorrect white background

I'm trying to export a large number of symbols in FLA files as Texture Atlases. I'm currently using JSFL to do this. The TextureAtlasExporter class will export the texture atlas, however, the image output when using the JSFL TextureAtlasExporter uses a white background instead of a transparent background. This leads to grainy animations with very poor quality around the borders. When I export the same symbol as a texture atlas using the Adobe Animate UI, I get a proper transparent background as expected.

Is there a way to get the JSFL TextureAtlasExporter to export texture atlases using a transparent background? If not, is there some other way to automatically convert thousands of symbols across thousands of FLA files into texture atlases?

Here is how I am using the JSFL TextureAtlasExporter:

var exporter = new TextureAtlasExporter();
exporter.filePath = folderPath;
exporter.autoSize = true;
exporter.resolution = 1;
exporter.optimizeJson = true;
exporter.imageFormat = "RGB8";
exporter.optimizeBitmap = true;exporter.exportTextureAtlas(symbolToExport, folderPath);

I have tried changing .algorithm, .imageFormat, .flattenSkewObject, and .optimizeBitmap. None of these affected the background color. I have also tried this on both Adobe Animate 2020 and Adobe Animate 2021. In both cases, the UI produces the correct transparent background, and the JSFL function produces the incorrect white background.