This is the only place we have noted separation with this engobe and body. The engobe was gelled and the piece was drained upside down. So this edge was thicker. The separation occurs to the convex contour a couple of mm down (and breaks away to that point). The engobe is more plastic that the clay and thus shrinks more. Normally the extra stickiness from the extra plasticity is an asset, but not always.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.