Perhaps I have missed the examples you cited.  I just checked §1.199A-0 Table of Contents in the Prop. Regs but didn't see any related to QBI items attributable to a trade or business.  The only emphasis that was placed on SE-tax at that time was that the QBI deduction does not reduce net earnings from SE for the purpose of SE-tax.

Could you clarify where in the Prop. Regs I can find the example so that I can take a second look for my own understanding?