Gimp 2: Brush tool (and others) are full-hardness like pencil tool

This is hard to explain as I don't know the proper terminology, but basically the usual behaviour of the brush tool is to mix/fade it's edges with the background, but it doesn't do this for some of my images (others work fine). This: is a picture to show what I mean. Other tools that would also have a fade/blur effect, such as the text tool and eraser tools also have this broken behavior. This is the image I was trying to edit (here is a non-hotlink if danbooru blocks you).

Anyway I'm just wondering if it's a settings issue that I accidentally changed, or something about the image, either way I want the old behavior back.

Thanks.

Solution

I solved it, the image was in "Indexed Mode", switching to "RGB Mode" in image > mode solved the problem.