I'm having an issue where I'm getting reflections from my pyro volume on a matte object. I've tried using a Render Geometry Properties and a Background Plate to matte it, but both have the same effect.

Maybe it's showing through the matte. I'm not sure what it's doing honestly, but it shouldn't be there

I just need a clean black hole for compositing.

That doesn't seem right...
I'm unable to repro with my test cases. Can you please file a bug with your scene? Thanks.