It seems like the tax logic is different in the edit sheet from when it is saved on the page. The below test case will produce you a 0.01 penny tax pre-save, and a 0.00 penny tax post-save. If you click between them the value changes:

Line item 1: £10, tax @ 0.04%

Line item 2: £10, tax @ 0.04%

Result pre-save (correct) is 1p

Result post-save (incorrect) is 0p