2024 Roth IRA conversion but F8606 not done

Trying to make sure I follow the fact pattern here. Are you saying:

1. There was no basis / nondeductible contributions (so 8606 Part I would be blank), and
2. The 2024 1040 return was filed with the 1099-R (so the tax calc is correct), but
3. The 8606 was missing (so the whole return was incomplete, and the IRS doesn't have log of the basis if withdrawals are made pre-59.5)?

If that's the case, I'd recommend to the taxpayer that they amend.

Sidebar: 8606 standalone is only for when there's not a separate 1040 filing requirement. I know of one case where it was rejected and tax pro was told to amend the 1040.

We can't force taxpayers to amend, but I lay it out for them that if they ever want to tap their Roth early (after the various 5yr rules), they will potentially face 1) more tax/penalties, 2) confusion with IRS inquiries because of mismatched records, 3) increased fees from you (you can't quote how high they'll be because who knows how high your rates will get in the year you have to fix this).

And they might get IRS correspondence sooner anyway because the 5498 won't match the lack of an 8606. I highly doubt there's AUR on those fields, but you never know, and I wouldn't make guarantees about it.

I don't try to scare them into amending, but I lay it out bluntly so they can never say "you never told us this could happen!"