I've been struggling with texture painting in 2.8 since its release. It always seems to have some little niggly thing that 2.79 had ironed out. In this case, the display of the texture - and when I paint it - is very bad. You can see in my image that the texture is fine, but how it is being displayed on the model is not... anyone know what's going on?
Switching to LookDev mode seems to fix it, whilst plugging the texture directly into the material output node in order to get flat colors unaffected by lighting.