Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
of the top of my head I would look into anti aliasing setting for selections, that will mess with the edge.Also you can go into refine edge:https://www.photoshopessentials.com/basics/access-refine-edge-photoshop-cc-2018/pierre
There are a couple reasons why this can be happening.1, when manually trying to erase or delete the softness on the edge, it's possible that the layer it's self is locked. Therefore, maintaining any pixels. Contracting by 15 pixels, should tho, show some new change. like the layer is locked, and you delete from it at 15 pixels, it should then not be deleted, but show the background color you had in your color window. So thats not it.2, You could have a layer activated...and accidentally had click a short cut key that would change that layer mode. Or maybe you bumped the layer mode with the mouse or pen tool. I often accidentally change mine to dissolve. (I hate that). But none of the layer "modes" have fuzzy or blurred edges. Just color changes.It would have to be something that maintains an "affect in styles". Affects are applied/found in the layer it's self. Inner glow, inner shadow, drop shadow, stroke all add something to the inside or outside depending on how you change the settings. That would be my thoughts...but you should see this then on your layer. If you don't see it, perhaps it is groups within another layer that goes have the affect.There are also times when I am working in layers...(and if I click on the layer), then my spot color seps will show visible while in RGB, showing over top of my layers. (Just a preview). So anything I did while in layers, like trying to take away a blur, would have no affect. Well, it does, but it's not affecting the "layer". Since the spot color is not active, due to my working in layers, it does not get affected either. It's jsut showing. So it would make you think what you are doing, is having no affect. It would, if the right "thing" was activated.Hard to figure out. Like teenage girls.