Box 7 code P on the 2018 form (indicating a 2017 taxable distribution) resulted in TT 2018 saying I needed to amend my 2017 return. However, in TT 2017, when I go to add a 1099-R, code P indicates a 2016 distrubution. So TT 2018 won’t allow me to add the 1099-R, and TT 2017 with code P indicates an incorrect 2016 distribution.
When you enter the 1099-R in your amendment return in 2017 there should be a follow-up question "Which year on Form 1099-R?" when you select 2018 TurboTax should include it correctly.