"they included conversion as a rollover 1099-R with code G"
You are stating this as if these are synonyms.
Rollover is the movement of funds from one account/plan of a type to a similar type account/plan.
Conversion is the change of the funds, as a move from pre-tax and sheltered to tax-exempt. That would be a taxable event and that's why the term "Conversion" applies.
You stated the destination was Roth IRA. What you need to confirm is if the funds came from a Roth 401(k) or a Deemed Roth inside of a 401(k). Because if this is really what happened:
"from their 401K to a Roth IRA account"
Then the instructions were incorrect and someone overlooked that this was Conversion. For instance, using the word Rollover, and not Transfer, implies your client handled the funds in between. If so, and they had a distribution check handed to them from a 401(k), then handed it to a new brokerage to "open a Roth for me with my employer funds" then they stated it wrong, and that would explain why the 1099-R is not coded for what really happened.
The issuer of the 1099-R only knows what they knew for money Out.
The recipient of the funds for a conversion or rollover only knows what they know for money In.
And on either side of this, things still go wrong.
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