Based on my own experience, if your 2019 over-contribution (due to employer ADP testing failure) was refunded to you before April 15, 2020, then you will receive a 2020 1099-R in January 2021. You would include the 1099-R in your 2020 return (if code 8 is used in box 7) and it will be added to your income and taxed as regular wages. TurboTax added my 1099-R to my regular W2 wages. This is how I do it, as my employer's plan fails ADP testing every year and I will receive a 1099-R every year.
There is guidance out there that says you should include the over-deferral amount in your 2019 wages. You may opt to do that, however, you will definitely receive a 1099-R for the year in which actual distribution occurs. Then you will have to explain to the IRS that you already reported the 1099-R in the previous year.
