I don't have any wisdom to offer but could toss in some thoughts that may not be worth 2 cents.

I agree with Rick that allocating SEHI to QBI of S corp would be a double whammy.  On the one hand, SEHI ($10k) is already accounting for as wages ($50k + $10k = $60k) and, thus, reduces the K-1's QBI ($100k - $50k - $10k = $40k).  And if we go by the plain reading of §1.199A-3(b)(1)(vi), that same SEHI ($10k) will be allocated to offset the QBI ($40k) on the individual's 1040 and artificially deflate the QBI to $30k ($40k - $10k).

Agree also that partnerships could have the same problem by virtue of SEHI being part of a partner's guaranteed payment but, unlike S corps, they could remediate this anomaly by forgoing the deduction of SEHI and treating that as a reduction against the partners' distribution (although the partnership may not otherwise take this position if not for §199A).

§1.199A-3(b)(1)(vi) reads [emphasis added]:
(vi) Other deductions. ***GENERALLY***, deductions attributable to a trade or business are taken into account for purposes of computing QBI to the extent that the requirements of section 199A and this section are otherwise satisfied. For purposes of section 199A only, deductions such as the deductible portion of the tax on self employment income under section 164(f), the self-employed health insurance deduction under section 162(l), and the deduction for contributions to qualified retirement plans under section 404 are considered attributable to a trade or business ***TO THE EXTENT THAT THE INDIVIDUAL'S GROSS INCOME FROM THE TRADE OR BUSINESS IS TAKEN INTO ACCOUNT IN CALCULATING THE ALLOWABLE DEDUCTION***, on a proportionate basis to the gross income received from the trade or business.

If we are to pick on words and consider the economic reality underlying the regulations beyond a plain reading, wouldn't we say that the general statement ***may*** not apply to SEHI of S corp 2% shareholder employees to the extent the guaranteed payment (inclusive of SEHI), which would otherwise be considered gross income under §61(a), is ***EXCLUDED*** in the calculation of the allowable deduction pursuant to §1.199A-3(b)(2)(ii)(H) (likewise, subparagraph (I) for partnership)?

On a different note, it is difficult to decipher what the phrase "on a proportionate basis to the gross income received from the trade or business" refers to.  My best guess is that it was poorly written but may conceivably pertain to situations where these specific deductions are attributable to multiple qualified trades and businesses, in which case, a ratable allocation among these trades and businesses would be warranted (as it is computed within at least Lacerte/PTO and probably PS).  If this was indeed the intent, I still don't understand why references are made to "gross" income when the allowable deductions are generally determined based on "net" income from these activities.

FWIW, I have seen only a few complaints on the internet and learned from other forums, at the very least, that Drake is also allocating SEHI to QBI of S corps, pending further guidance from the IRS.