"In 2025 I rolled over my entire 401(k) into a regular IRA at the same institution (not a taxable event). In a separate transaction in 2025, I transferred a portion of that regular IRA into a Roth IRA at that institution (a taxable event).
The 1099-R I received combined the two transactions and reflected the total 401(k) rollover in box 1 (not all taxable), and the Roth transfer in box 2a (taxable)."
That's impossible. The 401(k) and the IRA are separate accounts, so the payer must issue a separate Form 1099-R with respect to each of these accounts.
What the Form 1099-R likely represents is a rollover of a portion of the 401(k) to the traditional IRA and another portion of the 401(k) directly from the 401(k) to the Roth IRA, both portions reported on an single Form 1099-R with code G in box 7 and the IRA/SEP/SIMPLE box not marked. Given that, I suspect that the entire amount distributed from the 401(k) was mistakenly deposited into the traditional IRA rather than being properly split to the the separate destination accounts. If the entire amount distributed from the 401(k) was deposited into the traditional IRA, it would seem that the financial institution made an error either in making the transfers or in how they prepared the Form 1099-R.
Did you receive a Form 1099-R with code 1, 2 or 7 in box 7 with the IRA/SEP/SIMPLE box marked? If not, then you are mistaken in that there was actually no subsequent Roth conversion from the traditional IRA. Because TurboTax is limited in reporting only a single type of destination account for a code-G rollover, the Form 1099-R needs to be split into two, one for the portion rolled over from the 401(k) to the traditional IRA, and another for the portion rolled directly from the 401(k) to the Roth IRA.
