If the employer withheld funds from your pay and deposited those funds into the designated Roth account in the 401(k), which can only be done pursuant to your deferral election, the employer is required to report the amount with code AA in box 12 of your W-2.
Certainly filing inaccurate forms with the IRS is problematic. It also creates a real possibility of the employee filing in inaccurate tax return.
