there are two limitations for the QBI deduction. the QBI itself and taxable income.
QBI must be reduced for contributions to qualified retirement plans to the extent attributable to the trade or business (reg 1.199A-3(b)(1)(vi)
the contribution reduces taxable income but the conversion increases taxable income with no offset to the self-employment income for QBI purposes.
it gets more complicated if you SE income is from a specified service trade or business. Reg 1.199A-5 because the phase out is computed differently. (it's actually referred to as a phase-in because there is a phase-in of the eligible QBI reduction)
