the TransferRIP has different ways to rasterize images.
The raster options for light/dark textiles are programmed to remove the respective parts of the design which correspond to the color of the selected garment (dark/light). Since white is a special color that brightens-up the image it will not be rasterized.
If you wish to rasterize the white color as well, you have the option of using the “micro-mask”.
This option does not differentiate between the colors and places a complete raster-mask above the complete image.
btw to help colors keep their colorfulness even rasterized and on dark shirts, try to increase the brightness of the image (or color) before you print.