Any engine being made of material that can rust does rust, ie on the head surface where the gasket sits.
Additional flexing of the gasket takes place due to the piss poor design of the head on the early ones ie 2wd heads made by cosworth, hence the redesign of the waterways, and more particularly the support around the water holes, changing from a long slit on the 2wd to a series of 1/2" holes on the 4wd.
tabetha