Default Gray Matte

[code000000]

When using the 'Export PNGs into PDF' option, shapes and placed PNGs export with a gray matte. This only happens when there is no background color behind the layers with transparency, so when designing on a "white" background. I have to go back and add a white shape to all of my artboards in order to mitigate this.

I'm wondering if you could make the default matte white instead? Thanks!