As soon as I saw that glue up and additional riser, I knew this was bound to happen. You should have glued in a powerlam between the osage and bamboo. That would have stiffened up the handle area, preventing bend in the handle that will result in popping handles. Not much you can do about it now; it's too late to add a power lam. Your best bet is to glue up many THIN layers of wood (under 1/8") and to build up a handle like that. Still no guarantee...but the current riser must come off first.