Tax Not Added to Total if Tax is Removed then Added Administratively

If I take this order

Coupon 50OFFFIRST -$50.00
Subtotal $100.00
Subtotal excluding taxes $50.00
Texas Sales Tax $4.13
Order total $54.13

then change the status to In Checkout (admin/store/orders/34/) and then remove the tax in (admin/store/orders/34/edit) it removes it and then recreates it:

Coupon 50OFFFIRST -$50.00
Subtotal $100.00
Texas Sales Tax $4.13
Subtotal excluding taxes $50.00
Order total $50.00

Everything is great except that the tax is inserted prior to the Subtotal causing it not to be included in the Order Total.

I am using the latest 7-x dev release of ubercart (2012-Apr-12). If you can give me some pointers as to where to look, I can see if I can fix it.

Thanks,

Alex