Withheld Amount is Not A Conversion
Any amount withheld as tax from a Roth IRA conversion is treated as a regular distribution.
Example:
John request to have the 401(k) balance of $100,000 converted to his Roth IRA. He instructed the plan administrator to withhold 20% for federal tax. As a result only $80,000 was credited to the Roth IRA and 20% was remitted to the IRS as a pre-payment of tax for John.
The $20,000 that was withheld for tax is treated as a regular distribution and is subject to income tax and the 10% early distribution penalty on any taxable amount. The 10% early distribution penalty does not apply, if the conversion occurred when John was at least age 59 ½ of if john qualifies for one of the exception to the penalty.