Weird splitting of discount across all items and taxing discount

Great plugin!

We have applied a $10 discount every time product B is in cart with Product A.

If 10 of Product B & A are in cart, it should be $100 discount off the 2nd item total.

This is happening and working well – in cart and checkout. The order email admin gets is accurate also.

However – in the admin back end
a) it randomly splits that discount across 3 products …
b) AFTER it removed 10% tax (Australia GST) from the discount.

• How to have the discount to not be pre-tax?
• How to only apply it to the 1 line?

Not sure why it is randomly splitting it.

Any insight would be great thank you.

screenshot in dropbox here