Editing an alpha passes

Hi guys,

I'm not even sure if this is the right place to ask this question but it's giving me Fusion confusion either way.

I have an image sequence rendered from my 3D app. I'm been hand painting some minor fixes in the beauty pass in photoshop and saving over my .exrs from photoshop. Works just fine. However, if I touch anything in my alpha pass in Photoshop, Fusion no longer recognizes the file. I'm masking out my edits and then flattening my images before save.

I'm cool with finding other ways around this but I'm looking to understand what is causing the conflict.