I am wondering about how you are pressing the seams. I don't have this problem, unless I handle the piece excessively. I press my seams to the side. If you press the seams open perhaps this is why you have a lot of issues with the seam thread coming apart on the cut end of the pieces.
Pressing a seam open is actually puts stress on the sewn pieces on the cut ends, when you don't have a back stitch to stay it. Pretty much as if you would gently try to pull apart a seam you are in the process of ripping out to re-sew.
I strip piece a lot, sewing multiple strips together (sometimes 2 up to 5 strips in a strip set) and then cutting multiple large triangle pieces from the strip set, or other shapes and I don't have this issue. My machine is set at 2.5 for the thread length. I always press my seams to the side.