This is unfortunately a common problem with Clown kniives (Chitala chitala).
This has nothing to do with the Stingray it is an unfortunate thing that sometimes happens when these Knives lunge at their food as they do, the tongue actually becomes dislocated and protrudes through the soft tissue of the lower jaw and if left in that position the skin around it will attach itself making for a permanent disfigurement. The Knife will continue to feed just doesnt look the same.
Now I have tried to rectify this problem with these bony tongue fish many times but never got the tongue to take up its normal position, in most cases I had to anaesthesiaswith the fish with Clover oil and literally remove the tongue with sharp sterilised nail scissors. Now this might sound gruesome but its an easy process, you literally open the huge mouth taking away one third of the tongue, there is no blood involved its nothing more than a white stump of semi hard bone, the lower jaw skin will nit back together but the process is best done asap before the skin fuses around the tongue, don't worry about the hole which is left this will heal on its own. As soon as the fish is returned to the tank you need to ad a half ounce per gallon of salt to ensure the wound doesn't become infected.;
PLEASE NOTE
The fish needs to be places in a shallow dish outside the tank to put it to sleep, I mix 2ml of Clover oil to a couple of gallons of water, mix it well then place the fish in it.
Wait for the fish to roll and become limp (DO NOT LEAVE IT IN TOO LONG) usually a minute or two will see it lose consciousness, remove it from the water and wrap the whole body in a damp towel, open the mouth and push from under the jaw to get the tongue to point upwards, there will be resistance this is why it has to be removed (last third) don't forget.".not the whole tongue"
Once this is achieved you must then support the fish in a normal position whilst waving water towards its gills or better place its head approx 20cm away from some kind of flow,head on of course, been a fish that breathes atmospheric air it is important to keep lifting the head out of the water so the fish can take air from the surface, it will soon start to swim, hold it until it can resume its normal position in the water. I have performed this operation 3 times and all were successful .
Your welcome to Pm me if you need further info or if anything I have wrote isn't clear.
All the best.