Express router module losing params

Question

I'm refactoring my router in express app, and I'm facing an issue regarding the params of the query. I splited my routes into multiple files, and created router modules for each.

These routes are registered in an index.ts :

import focRoutes from './foc';
import fedRoutes from './fed';

const router = Router();

router.use('/foc', focRoutes);
router.use('/foc/:foc_id/fed', fedRoutes);

export default router;

Then defined :

// foc.ts
const router = Router();

router.get('/', () => {...})
router.post('/', () => {...})

export default router;

// fed.ts
const router = Router();

router.get('/', () => {...})
router.post('/:fed_id', (req) => {
  console.log(req.params.foc_id); // <- this logs `undefined`
  console.log(req.params.fed_id); // <- this successfully logs the id
})

export default router;

The issue is that the req.params.foc_id set in the index.ts becomes undefined once in fed.ts. Previously I had all the routes in the same file, and doing this worked well :

router.post('/foc/:foc_id/fed/:fed_id', (req) => {
  // Can use `req.params.foc_id` and `req.params.fed_id` without issue
});

Since I put the :foc_id param in the index.ts file, it's not available from the associated route file. If I move back the param to fed.ts it works well

router.post('/:foc_id/fed/:fed_id', () => {}) // <- no issue

Is it normal behavior to lose the param along the chain of router modules ? I couldn't find anything about this issue and don't know what to do.