Then having any tax withheld would be senseless. Earnings are taxable in the year that the contribution was made (2018), but tax withholding can only be applied to the tax year that the withholding occurred in (2019). In other words, 2019 withholding cannot be applied retroactively to 2018 to pay any 2018 tax due and can only reduce your 2019 tax liability.
You probably will not get the 1099-R until Jan 2020 which will require amending 2018 unless you know the exact amounts of returned contribution and earnings - then you can report it now with a dummy 1099-R if you have all of the payers information that will be on the 1099-R. Then you will not need to amend. (Some payers will issue a 1099-R early for this - ask them).
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
