I used to do a lot of inlay /onlays with feldspathic, and they would often crack on glazing if there was a thin section joined to a large. It was as they cooled so if you were quick you coud devest them before the stress propogation started to show itself. I have not seen that in normal veneers , but as Grantoz said look to CTE changing over multiple firings. Maybe check compatabiity if you are new to them , or have changed porcelain or refractory material.