Line items are taxed multiple times

the use case:
- shipping, tax and discount line items (from uc_discounts)
- several tax rates applying to different discountable product classes and to the corresponding discount line items.

Problem: the line items get taxed cumulatively at every rate which is set for these line items.
How to apply only one rate to a specific line item rather than all configured rates i.e. a specific discount for a specific product?

The tax rules setting allows to select :
- product types
- line items type regardless the selected product type or the specific line item name

So, if an order has :
- product A taxed at X% - discount A
- Product B taxed at Y% - discount B

then discount A and B are both taxed at X%+Y%

Thanks for help.