Well, If it chips off at that point, it's probably indicative that it was never bonded....so you'd want it off anyway. If the chip is at least a 16th deep, you wont have to worry about the feathered edge (although I would not use sanded grout 1/16th thick...that's too thin...you're back to plaster)

The problem then would become an irregular transition line (from old to new)that would probably not be very esthetic. I cant think of a way to solve that.