Weird "feathering" problem when masking

Hello, I'm a relative beginner at fusion, coming from AE so this question might have an obvious answer but I've been searching the internet and can't find what's wrong...

My comp consists of a source clip in ProRes which I'm trying to add a couple EXR sequences onto. These are 3D assets I've rendered from Blender in linear.709, I'm trying to blend these assets into the real scene with a mask but as soon as I feather it one of two things happen:

If I have the mask connected to the EXR sequence and try to feather the edge of it there's a soft white "glow" around the mask.

If I connect the mask to the merge node there's the same weird "glow" around the border of my mask but it shows as a transparent checker-background instead.

See image to view the node setup and problem.

I would not be able to help you before seeing the actual project in front of me but what I would recomend is to create a bespoke find edges using BITMAP node and add it as a mask to a BLUR node. It's a very neat and pro solution.