Did you cut a triangle out? That would leave too much fabric at the point. If so you should have just cut a vertical slit like this. The first image is the shape you are aiming for, which is basically the fold lines. Then you cut a vertical slit and fold them back. This gives the fabric a place to go. You can then trim off any excess. if I were doing this for garment construction, I would first sew a line of stay stitching along the black line with matching thread then cut. This will reinforce the point area, but for a quilt there really is no stress on it so it's probably not necessary.
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