Having an excess Roth 401(k) contribution that is unable to be timely corrected can create a real mess with regard to basis tracking and determining the taxable amounts of distributions. A substantial complication is that, in such a case, the employer might not track this excess and attributable earnings. To comply with the requirement that the distribution of the excess and attributable earnings is taxable you might need to do the tracking yourself. If so, any Form 1099-R that reports a distribution from one of these Roth 401(k) accounts will not show the correct taxable amount and the employer will not be able to correctly track your contribution basis.
You might consider finding out if the plan that you notified about the excess will do the necessary tracking so that they can report the correct taxable amount and track the basis properly when you take a distribution of this excess and attributable earnings after becoming eligible to take distributions from this Roth 401(k).
