This was/is a fairly common failure with this seat stay design. It's very strong when the stay (or the angled cap) comes up higher so the joint is where it's relatively thick. But when the joint is near the tip, there's less cross section and flexing leads to metal fatigue and failure.

It is repairable by brazing, and while that's being done, the opposite stay should have some braze filled under the back to extend the support out to where the cap is thicker.

Problem, is it isn't cheap, and you have to decide on what kind of repainting will be acceptable.

Just about anyone familiar and skilled in basic frame building practices can do this for you. If you want a baseline quote send the photos to Yellow Jersey outside of Madison Wisc. They do quality work at very fair prices and would be my go to guys if the price was in line with my budget.

BTW- you can also get decent work for stuff like this at a motorcycle speed shop or out in farm country where guys know how to keep stuff together on the cheap.