Denoiser significantly affecting the alpha channel

[marksmanguy]

When I use render region, the denoiser affects the alpha a lot. Makes a halo around the render region edge, as well as a halo around the object, Every time I need to go into photoshop and mess around with the levels to make it pure black and white. Am I doing something incorrect or is this a bug?

Some examples attached, and I used a curve in one of them to exaggerate the areas being affected.