2.1.11 [ISO32000-1] Section 220.127.116.11.7, Type 6 Shadings (Coons Patch Meshes)
The specification states:
Patches can sometimes appear to fold over on themselves—for example, if a boundary curve intersects itself. As the value of parameter u or v increases in parameter space, the location of the corresponding pixels in device space may change direction so that new pixels are mapped onto previous pixels already mapped.
Self-overlapping meshes are not supported.