My Ranchu Tiger has a similar problem, hole in Wen, sunken. still happy, no signs of stress. Tiger is orange so I can see the tissue around it is pink. it would look white on your color fish. Tiger has a great appetite too
I treated Tiger originally with betadine swab bi daily over 10 days. The pink color went away and it stopped getting worse.... but he has had a relapse.
If you can find it there is Hikari bio bandage that is a great start for any soft tissue issue and better alternative to betadine. I just cant seem to find it anymore