Its the elements inside the grip...if all the electrical is ruled out.
Mine stared that way, then one stopped working all together...I checked my resostat too, and wiring...all checked out.
Im sure it was someone on this site that told me to check the grips themselves, and thats what it was. Especially if they are stock
If you buy a new set, you can test them quickly by tapping in a splice down near the grip itself, and then connecting the grip(s) just maretted, no solder...obviously if it heats up nice, and responds to changes in the reostat, you know you need to change em.
If not, they'll still look nice, cut off the small section of wire you tapped into, and return them for a refund...no harm no foul and you'll know one way or another