To have $12,000 on line 17 you must have $12,000 on line 1. $12,000 on line 1 implies that you have an excess contribution traditional IRA contribution of $6,000 reportable on Form 5329 and subject to a 6% excess contribution penalty. Also, an excess contribution to the traditional IRA is not really eligible to be converted to Roth. Doing so results in a failed conversion, so reporting a $12,000 conversion (on line 16) would also likely be improper.
The only way that the Roth IRA custodian can legitimately make a return of contribution from the Roth IRA is if it treats the excess amount in the Roth IRA as having gotten there as a regular contribution, not a conversion contribution.
