Very common
This is very common on all hunters, and most other sailboats that used a cored rudder. As a marine surveyor, about the only boats I have seen that don't have this problem are those that had solid laid up rudders such as old Pearsons. I have rebuilt a few of these, and have discovered that the problem seems to be in the construction process. If you cut the skin back over the delaminated area, you will probably see that the foam was incompletely filling the cavity, and voids are present. Fill these up with resin/cloth, then lay a new piece of skin on with mat and resin. Bottom line is.... about any one you look at will have this problem, unlkess someone has already fixed it. Good luck, Dave