Private field bug

Hello, I switched from babel to tsup today for my library and got this, I use private fields and here my tsup config:

```ts
import { defineConfig } from "tsup";

export default defineConfig(options => {
return {
minify: !options.watch,
entry: ["src/index.ts"],
splitting: true,
sourcemap: true,
clean: true,
dts: true,
format: ["cjs", "esm"],
platform: "browser",
target: "es2018",
bundle: true
};
});

```