The Roth account in your 401(k) is not a Roth IRA.
For distributions from the designated Roth account in your 401(k) to be entirely nontaxable, the distribution must be made after the 5-year holding period has been completed and after you have reached age 59½ (or are disabled). The Form 1099-R reporting the distribution from a designated Roth account will have code B in box 7 (perhaps accompanied by code 1 or code 7) and should show the correct taxable amount in box 2a.
