VAT doubling on checkout issue

We've come across a really strange issue with the VAT calculation in shopping cart.
if we add a taxed product to cart, the tax amount is shown correctly.
If we then add a Non taxed product to cart, the tax on the original taxed product actually doubles.
Add another non taxed product and it again adds the original taxed product amount of VAT and so on.
The order total is always correct. It's only the VAT field that shows an incorrect amount.

If however, we first add a non taxed product to cart and then a taxed product, the VAT display is fine..
I'll send you our dev server details to your email if you'd like to see what's going on yourself.

Thanks Nicolas. Cant believe the fix was so simple. We literally spent hours banging our heads against the wall trying to diagnose this.
Anyway it now works which is the main thing.

I'd love to be able to update Hika to the latest version but when we tried (several time), it completely destroyed our layout and menus. We cant figure out why.
I sent you an email last week asking if you guys could try to update the 1.5.6 version on our dev server to diagnose the issue. I'm happy to pay for this as I realize it may a problem on our end.
Is there someone I could contact to set up a date/time/cost for this?