I don't really know, but my guess from my experience would be just not enough soil in a grow bag for them. In my previous location, with little space, I tried growing potatoes in a 50 gallon trash can, cut in half crossways to not be so tall. Put the seed potatoes close to the bottom and then kept filling it as the plants grew. Doing that, I got potatoes, but they ranged in size from marble to golf ball. Now that I have more room, I am growing potatoes in the ground and getting full sized potatoes.
Dying back is what potato plants would be doing this time of year. I think there is nothing you can do for those plants. Next year try to find a way to give them more room/ soil.