In 1.3.8, the label "Sales Tax" would be used because no actual tax applied, i.e.

Sales Tax: $0.00

In 1.3.9d, a tax which does not apply is displayed:

FL TAX 7.0%: $0.00

I reviewed a few files and didn't see an obvious root cause. A patch would be to use TEXT_UNKNOWN_TAX_RATE in ot_tax.php if value is 0.