I contributed $6000 to traditional IRA in March 2021 and another $6000 in Aug 2021, thinking that the first $6000 counts toward the 2020 limit. Today, I realized I forgot to report my 1099-R on my 2021 tax. While I was amending it, I found that 1099-R says my backdoor IRA distribution has $12000 taxable even though I contributed to traditional IRA with after-tax money. When I entered 1099-R into turbotax, it says "Good News: You don't owe extra tax on this money", but after I clicked "continue", the federal tax due went from $0 to $2100. It seems to think I have over contributed. Where did I make a mistake?
