I'm in no way an expert so hopefully someone else will also include their thoughts... but my first thought was a bacterial infection called ulcer disease (also known as Hole-in-body disease or Red sore)
Common disease signs are ulcers or “holes” in the body, red sores, or a red mouth. When it's more advanced they can swell up and have a "pine cone" appearance where the scales kind of seem to jut out. The eyes can bug out too... but it would be hard to tell on your particular goldfish.
It's said that this illness is generally triggered by stress or injury...
when you saw water conditions are great... what are the levels? (especially ammonia, and Nitrates...) also... do you have plastic plants in the tank? Plastic plants tend to rip off the protective slimecoats on fish which can expose them to infection. That's why I always use silk plants (If I don't have real ones in my tank.)
from what I read it is something they can recover from... there are a few ways to handle it... from treating the wound to leaving well enough alone, to isolating in a treatment tank...
but since I'm only guessing what it could be... I don't feel comfortable giving you direction on treatment. I'm only giving an idea of what it COULD be. I'll leave the diagnosis and treatment recommendations to the more knowledgable.
