Unit rounded invoice calculation error

[Mattia]

Setting: swidish round by adding an invoice line. Tax rounding unit 0.05
Tax on products 2.5 %

Sometimes the addon fail calculating the rounding import or the balance do not correspond the total of the invoice

sequences working:
Create sales order => sell 1 product with price 10.858 => confirm sales => create invoice.
Result:
Product 1 for 10.858 = 10.86 chf
Rounding 1 for 0.02 = 0.02 chf
Subtotal = 10.88
Tax = 0.27
Total = 11.15
Balance = 11.15

not working sequence:
create new invoice => sell 1 product with price 10.858 => validate invoice
Result:
Product 1 for 10.858 = 10.86 chf
Rounding 1 for 0.02 = 0.02 chf
Subtotal = 10.88
Tax = 0.27
Total = 11.15
Balance = 11.12 (WRONG: because of this the payed bills do not get automatically matched)

If after creating the invoice I update the tax by clicking right next to the Tax import, and I then validate the result is right, and the total is the same as the balance: 11.15 chf

Another anormal behaviour is when I sell 5 products priced 10.858. This lead to a total of 55.65 included 2.5 % Tax and should not be rounded. Nevertheless the addon adds the rounding line with quantity = 1, price = 0.01 chf but strangely amount = empty.
This leads to a total of 55.65 chf and a Balance of 55.65 chf which is right but on the invoice the rounding line should not be there.

Is this a bug or am I doing something wrong?

Thanks