Paint is either a solid or a tint. A watercolor stains the paper, but an oil or acrylic sits on top of it. So as you thin it down to mmake highlights and such the pigment gets thinner distribution and so it can break. Some media encourage that (crackle textures, for example) and others make it stretchier (flow improvers). But it’s still a thin layer of paint and if you put anything on it that has a solvent component, like a varnish, it can re-dissolve the outer layers and absorb them into the varnish and make them seem to “bleed.” It happens with ink on coarse paper, too – the liquid moves faster than it dries, and dropping water on it makes it dissolve again and run in weird directions.