ouch! that was a beautiful handle too.
a couple of possible things may have happened here. Since I'm assuming you have not put it in the dishwasher, it's not that.
it could be from too thin of a sealer on there allowing moisture in. Stress from clamping the handle in place to pull shots
or
It could be (and looks like) a natural crack since it looks like it's originating from a knot area, aggravated by the above possibilities. Unfortunately the only thing you can do is replace it. You could try putty but with the temp changes from pulling the shot will be enough to cause that putty to pop out after a little while.
I would recommend to continue trying to contact the original woodworker. if that doesn't work out, contact me, I'll take care of you.