It's seems likely that your deductible solo 401(k) contributions are reducing the amount on which your QBI deduction is calculated. That would make the reduction the tax liability for a deductible 401(k) contribution be at a rate that is less than your tax-bracket rate. Still, deductible solo 401(k) contributions reduce AGI even if they don't reduce your tax liability as much as they might otherwise, and there could be some benefit to having a reduced AGI such as lower Medicare IRMAA (for those who are subject to Medicare IRMAA).
Keep in mind that deductible 401(k) contributions are deferred income. You'll pay taxes on the money eventually, at whatever marginal tax rate that you have when you receive the distribution. If your plan offers a designated Roth option, you might consider contributing to that, since gains will be tax free once the requirements for qualified distributions are met.
