I had a similar situation on a hickory backed ipe bow I made. It didn't really have a knot, but a swirl in the grain. The knot was actually a little farther down in the piece of hickory. I took a change and made the bow anyway. It shot great till about arrow 160 or so....then a large splinter popped up with the point right at the swirl. I glued it back down and wrapped it with artificial sinew and super glued it. It held up for several hundred more shots, but I noticed the crack was re-appearing the other day. I have retired that bow....at least till I get a chance to put a new back on it. Just my 2 cents.