The multires “reset” method might fix your issue if you find the behavior kind of buggy, but it looks like this artifact appeared as a result of rebuilding subdivisions.
It looks like this hole is the base of a multires “spike” artifact. Can you confirm by looking on the inside of the head for the point of the spike? Such spikes are an annoying multires bug but they can often be smoothed out as I did in chapter 3, lesson 10.
