My lions all have solid jaws, must to a to low res condition on mine.
The "hole" is on the bottom side of the lion fish mouth, hard to see unless you cheat a bit. The fish has to be facing (or going away) and tilted so you can see the underside. In the picture you can see the green of the coral through this hole. Actually under the right lion fish position there is a hole around the entire dorsal fin as well. Neither worth fixing in my opinion.