Didn't go so far as to cut into the rudder shaft itself, it wasn't exposed/visible through the foam when I cut down the length of the rudder.
of course, it is possible that water did get in and was wicked up, and that's probably what happened. Delamination occured where the shaft enters the boat.
I do a quick wash with hose after saltwater use, but I have a hunch that spot doesn't get a full washing, or at least as much as it should have.
Thanks for the reply!