Thanks for posting the additional info. There is nothing obviously wrong with your settings shown above. I assume that you did not modify the exported .png
atlas texture file after export, right? If you did, please note that some image editors will mess with semi-transparent PMA pixels, not retaining the correct PMA values.
Could you please send us both your problematic Spine project assets (including the attachment images), and a minimal Unity project that still shows this issue? You can send it as a zip package to contact@esotericsoftware.com (briefly mentioning this forum thread URL so that we know the context), then we can have a look at what's going wrong.