I have the same situation, but on the letter included with my deferral refund, it explicitly states that the deferral is taxable in 2019 and the earnings are taxable in 2020. On my plan's website, it shows the two 1099-Rs, one for the deferral with code P and one for the earnings with code 8.
Code P means it is a return of contributions from the previous year and Code 8 means it is a return of contributions from the current year.
When you file your 2020 taxes and you enter the one with Code P, that amount will NOT be added to your 2020 taxable income. You can test this yourself right now by making a 1099-R and selecting box P and keep going though the steps and if will ask if it is a 2019 1099-R or a 2020 1099-R. If you select 2019 (current year), you will see your estimated refund does not change. If you select 2020 (next year), it will count it as taxable income for 2019 as it is supposed to. So next year when you enter that same 1099-R again, it will ask you if it is for 2020 (current year) or 2021 (next year). You will select 2020 (current year), it will NOT be counted as taxable income again and you will not need to amend you 2019 taxes because you already reported it and paid the taxes.
