The zero in box 2a of the Form 1099-R indicates that the 401(k) plan believes that you requested a rollover to a traditional IRA, not to a Roth IRA. If you properly requested that the 401(k) plan make the rollover be to a Roth IRA, the plan needs to correct the Form 1099-R to show the same amount in box 2a as is in box 1.
If you did not specifically indicate to the plan that the rollover was to be to a Roth IRA, you yourself redirected the rollover to a Roth IRA and there is no reason that the plan would be expected to change the Form 1099-R. In that case you would need to include a substitute Form 1099-R (Form 4852) in your tax return showing the same amount in box 2a as is in box1. Note that including Form 4852 prevents you from e-filing.
