Rollup Version

2.78.1

Operating System (or Browser)

Linux/Firefox

Node Version (if applicable)

No response

Link To Reproduction

https://replit.com/@flying-sheep/rollup-repro-esm-from-ts

Expected Behaviour

rollup.config.ts gets compiled to ESM and imports <projdir>/node_modules/@mdx-js/rollup/index.js as ESM

Actual Behaviour

rollup.config.ts seems to get compiled to CJS despite target: ES2020 in the tsconfig.json.