We have a similar (same?) problem showing up for us when a group discount is applied to an in-state (taxable) order. The sales tax is undercharged.

For us, the problem is more hypothetical than practical, because the only group discount we offer is to dealers who are normally tax exempt. Mostly, this reply is to let the Zen wizards know that we had discovered the same kind of error cropping up during testing, on in-state orders when a group discount is applied. In case it helps someone in solving, here are the total lines from a test case:

TOTAL LINES DISPLAYED DURING CHECKOUT:
Sub-Total: $7.99
US Postal Service (Priority Mail): $6.60
Group Discount: -$1.20
NC TAX: $0.84 [wrong!]
Total: $14.23

SOME OBSERVATIONS:
Tax is charged based on the shipping address.
Shipping charge is taxable.
Group discount is 15% of the product sub-total.
Tax rate is 6.75% (for this order; rate varies by county).
Sub-total (product) plus shipping less discount is $13.39.
Tax amount should be .90.
The amount shown, .84, is actually 6.27% of $13.39.
If the sub-total plus shipping less discount had been $12.44, then .84 tax would have been correct.
Same error occurs in both 1.3.7 and 1.3.8a.