Reproduction: https://github.com/steinybot/bug-reports/tree/tsup/imported-ts-interface

volume.ts:

export interface DirectoryJSON {
  [key: string]: string;
}

index.ts:

import {
  DirectoryJSON,
} from './volume.js';

export { DirectoryJSON };

Outputs:
index.js:

import {
  DirectoryJSON
} from "./volume.js";
export {
  DirectoryJSON
};

Which blows up when you try and import it as clearly volume.js has no export named DirectoryJSON.