The only time my stitching comes apart is when I didn't keep the seams straight so the rows slip apart and I've learned to backstitch at the beginning & end of my rows before starting the final assembly into a quilt.
Chain piecing with no little tail left between the patches will also quickly unravel. My machine doesn't like the couple empty stitches between the patches so I've just quit trying to do it that way.
I would definitely change the way you are doing something else before changing all my patterns to 1/2 inch seams. Fabric is too expensive to eat up with seams.
Flannel can also be sewn with 1/4 seam allowances without fear of it coming apart if it is prewashed, pressed with a very hot iron and you are very consistent with your 1/4 inch. The prewashing, drying and pressing should shrink/draw it up enough to hold even the looser weaved (usually cheaper but not always) flannel together. Less than 1/4 inch anywhere in the seams and it will unravel enough to reach into the seam line and come undone.
Another idea is, Are you pressing your seams open? If your tension is loose and you are pressing your seams open, that could easily weaken the thread enough to pop seams.
Post a photo of the back & front sides of one of your finished blocks after you have pressed them so we can see what's going on.