Not that this is going to solve anything.
But If you test your water and everything is fine. And If you haven’t had any lawn chemicals or foreign substance accidentally introduced into the pond. No rapid changes in weather.
Then the only way to tell is to have a lab or someone look at, take blood, (not sure of blood can be checked after dead) and dissect.
I just watch a video of something similar. Girl had like 20 of some kind of fancy gold fish she was breeding. The all got sick. Showed signs of dropsy. Such as raised scales.
she sent 4 of the worst ones still live to a lab. They took samples and dissected them.
Even with all there equipment all the could tell her was. They showed signs of kidney failure. They had hardened Nodules all over their Kidneys. The nodules were from its immune system fighting something. Either bacteria or parasite encasing it to stop it. Which lead to fluid retention causing the scales to pop out and be inflamed.
So after all that they sent her a letter stating they couldn’t tell exactly what caused there death. But explained what they found and the it was either a parasite or bacteria. But could not tell what kind. And have her no definitive answer.
I found the video.
She is a pretty experienced breeder with gold fish. Her YouTube channel is pretty good. She has a shit ton of fish and pets
Fish are fish diseases are the same koi goldfish all types of carp.