Credit Note wrong tax calculation on shipping cost

Hi I found a bug when crediting an invoice, issue described as follows:

- I credit an invoice which originally contains a shipping cost of $15
- Before making any entry, the default value in the field "Credit Shipping Cost" is shown as $15
- And the tax value is shown as 3.45 and the Credit Note Total is shown as $18.45 (because tax is 23%)
- But I don't want to credit shipping cost, I only want to credit product. Hence I adjust down the 'Credit Shipping Cost' field to $0
- However, the Tax Value remains at $3.45 and the Credit Note Total shows $3.45, even all other fields in the Credit Note show zero.
- It is no use even I press Update
- I tried to re-enter a value into the Credit Shipping Cost to see if the tax value will change. So I input $10 back to the field. The Tax Value field does not change. But the Credit Note Total becomes $13.45.

This is very strange. May I know how I can solve the problem? Thanks.