If you don't have a non-deductible contribution then you don't have any basis in the IRA (assuming no prior basis in the IRA carried over), so the Roth conversion is assumed to come from pre-tax IRA money and will be 100% taxed. With the contribution on Form 8606, the conversion comes from after-tax money and will be determined to be 100% tax-free.
Suggest going thru Form 8606 to confirm, you'll see how the contribution and conversion are connected on that form and the calculation of how much of the conversion is taxable.
