So far it seems to be close to the end of the stave, I haven't stripped all the sapwood off the rest of it yet. I'll get to work on that and see if it keeps happening, here's a close up and a view further back with a dotted line showing the sapwood.
Never having done this before I'm a bit nervous about how far is too far. I think I've figured out what the "pithy" areas are talked about in Clay Hayes' videos, but is it a dealbreaker if you end up going past it? The outermost ring is well over 1/4" thick (at least at the very end of the staff, I doubt it's consistent all the way through).