it is possible that your seam allowance is not consistent in width as you approach the end of the side. This will cause distorted mitres.
Also.....as you are approaching the corner, stop about 2 stitches before you get to your 1/4", 1/2" or 3/8" or whatever your width of seam. This will allow for the fold just as the scant 1/4" will allow for fold in piecing.