Ok
You need to go to the customer card and in the invoicing section select the VAT Bus posting group field

In the dropdown list select Select from full list and the next screen will open

Click on the three dots button in red square and select setup

The next screen will open

The calculated tax in sales invoices relies on the VAT Business posting group (with whom I am making business) and the VAT Product posting group (what I am selling) so, in a perfect world, you need to use the crossing red squares

But, besides de customer, you also need to check the item that you are selling

Example
Customer is domestic but item is 15 (last image)

Same customer with item at 10

In sales order, related, statistics you can find

For the posted order, make a credit memo before the VAT PROD Posting Group change