2,4-D will kill the hades out of chufas. I can't understand how anyone recommends it on chufas. IF you are able to put out the perfect amount, it will kill the weeds and not kill the chufas, but that amount will vary from year to year and field to field. I'd guess that your chufas that survived were in better soil and probably in better condition than the others when you sprayed them. The rate you used worked in one field, but was too much for the other. I'd find another broadleaf herbicide before next year.

Good luck with your remaining patch.