https://irahelp.com/slottreport/10-penalty-roth-conversion-distributions-and-form-8606-today%E2%80%99s-slott-report-mailbag

In regard to the second Mailbag question, since the the distribution from the traditional IRA occurred in December 2019, even though the conversion to Roth was not completed until 2020, reporting the conversion on a 2020 Form 8606 would produce a nonsensical result; it’s impossible to report the beginning of this transaction on the tax return for one year and the completion of this transaction on the tax return for the following year. If the distribution isn’t reported at all on the 2019 tax return, that’s sure to trigger a CP2000 from the IRS with respect to the 2019 tax return.

See 26 CFR § 1.408A-4 Q&A 7 which clearly indicates that the taxable amount of the conversion is includible in income on the tax return for the year in which the *distribution* occurs. A distribution in 2019 reported on a 2020 Form 8606 would not produce this result.

Note, however, that the conversion contribution will appear on a 2020 Form 5498, not on a 2019 Form 5498.