The main issue is that the stars are saturated in the raw image, so they are in the stacked image too. There’s a bit more signal around the saturated pixels in the stacked image so the algorithm is able to find stars, but not in 8 bits.
You cannot convert the JPEG into 16 bits, or not in a meaningful way, because JPEG images are 8 bits.
So next time, shoot in raw, but also try not to burn the images, it’s not a problem if objects appear faint, with stacking you’ll get them.