This topic (seam coming undone) has been coming up fairly often. I've had it happen to me, and it wasn't because of too little seam allowance - I think maybe the stitches at the end of the block didn't hold. Perhaps the block was handled too much? I think I better start checking all my seams while pressing, or at least before I start quilting. I've also run into the problem when longarming charity quilts from guild members. So... you're not alone! If you can work an applique over the seam, that would be a wonderful solution. Otherwise, careful stitching, fusible on the back... whatever works. Take the time to undo quilting if you really need to. Your quilt is very pretty and extra effort spent now will be worth it in the long run. Good luck.