When you are quilting you need to lock the turn of the thread. Do this by doing a double stitch at the point. I am a sewer from the old school and when top stitching any item this problem will occure at times. Just do 2 stitches any time you need to change direction and you will not have this problem.
PM me if you have any questions on this.
I have used this for years and my quilting points are always sharp.