I have an idea. Do you have any deco that could do a little damage? Or a tankmate who may nibble? This happened to one of my goldies.
The nibbler was another same-strain goldfish. A little smaller, too.
If there's deco, run a finger over all of it to be sure it's smooth.
Sandpaper and a good rinse works for me to get rid of edges when I buy deco.
If it was nibbled/torn, just watch for fin rot. Oh, and if it does occur, try to do water changes regardless of how the water looks. Water changes solve lots more than seems possible.
The black dot... just watch it and use medicine.