This is the intended behavior. Because you made nondeductible traditional IRA contributions and Roth conversions in the same year, TurboTax uses Worksheet 1-1 from IRS Pub 590-B to calculate the taxable and nontaxable amounts instead of doing this portion of the calculation directly on Form 8606. This is denoted by asterisks on lines 13 and 15 of Form 8606.
