I have same question and came across this:
According to the IRS's guidance, Roth employer contributions to a 401(k) plan are effectively required to be reported as if the contribution had been made on a pre-tax basis, and then immediately converted to Roth. Which makes sense in that it doesn't require wholesale changes to existing tax forms or payroll systems, but does have an unintended side effect for self-employed owners of solo 401(k) and SEP plans who are also eligible for the Sec. 199A deduction for Qualified Business Income (QBI): Because the business's QBI is reduced by the amount of any deductible retirement plan contribution, the fact that Roth employer contributions are reported initially as deductible contributions mean that they reduce the business owner's Sec. 199A deduction, even though they don't actually reduce their taxable income (since the income from the Roth contribution is added back in the form of the "phantom" Roth conversion as required by the IRS's guidance).
Accordingly, business owners who contribute to a solo 401(k) or SEP plan and are also eligible for the Sec. 199A deduction may want to avoid making Roth employer contributions, even if their plan provider allows it. Fortunately, however, there is another way to maximize Roth contributions to a solo 401(k) plan that doesn't affect QBI in any way: If the plan allows employee contributions to be made on an after-tax basis, the business owner can simply make after-tax contributions (all the way up to the $69,000 total contribution limit) and convert them to Roth, which is a tax-free transaction since the contributions weren't deductible to begin with. And because there's no deduction for the contribution, it won't affect the QBI calculation in any way.
The key point is that, as business owners decide on their solo 401(k) contributions for the year, they may be unaware of the effects that making Roth employer contributions might have on their Sec. 199A deductions.
