I know this is like uncouth, but could you open it in devtools and show me the output?
I've gotten hundreds of comments from other people on the internet who were able to use it. I would love to help fix whatever this bug is so you can try it out too.
I've found LLMs come up with really good color palettes, too.
An interesting tool might be a mask tool, that let you highlight parts of the image to mark regions as weighted more highly in the clustering algorithm.
To be able to lock the palette would also be cool, but it would probably need some sort of dithering to be useful.