Also, since this distribution is eligible for rollover, 20% federal tax withholding would have been required. To complete the rollover of the entire gross amount of the distribution you'll need to substitute other funds for any federal or state tax withholding. Assuming that you do the rollover, when you enter the 2020 Form 1099-R while preparing your 2020 tax return you'll indicate that the distribution was moved to another retirement account and you'll indicate the amount of the distribution that was rolled over.
(Had you requested a direct rollover before the plan had to make the distribution on its own due to the balance being under the threshold for allowing the 401(k) account to maintained, mandatory tax withholding would have been avoided. At least 30 days prior to making the distribution, the plan was required to have provided information regarding this and other distribution options)
