Checkout: wrong calculation of tax when using coupon

When using a coupon the the tax keeps being calculated based the original price instead of the new price after the coupon is applied.

To reproduce
1) In the settings at the Tax tab:
1a) check the “Turn tax on”
b) Select “Product prices are tax exclusive – add tax to the price during checkout”
1c) Select “Replace tax percentage with product specific tax rate”
1d) Select “Apply tax when Billing Region is the same as Tax Rate”
1e) Add a country, for example “Netherlands” and add 21%

2) Create a coupon with 25% discount
3) Create a new product with price 100
4) Buy the product

At checkout
5a) make sure you select you select the same country as with 1e
5b) Enter the Coupon code
5c) Press the “Update button” behind the coupon code

Result
– Cost before shipping: 100, this is Ok.
– Tax 21,==, this is WRONG,
this should be (100 – 25%) = 75 * 21% = 15,75
– Discount: 25,== this is Ok
– Total price: 96, this is WRONG
This should be: 75 + 17,75 = 90,15
BTW, in version 3.8.12.1 the calculation is Ok !!

I created a complete new test site, with fresh WP 3.9.1 installed
I added the WP E-Commerce 3.8.14.1 plug-in
No further plug-ins!

https://www.lightroomstatistics.com/ontwikkel7