Tax Calculation Issue for Sales Invoices

Hi,

The customer is asking me to understand why the tax calculated is incorrect for a sales invoice. It's quite strange because the tax classification code is zero, but the tax is still being calculated, and I suspect that some tax rules are failing. In fact, in the zx_lines, the only column of 'result' populated is the PLACE_OF_SUPPLY_RESULT_ID, while other columns are empty. If I run my personal query, I only get the place of supply rules.

Can someone provide a query to help understand why the old tax rule that usually works for this particular case isn't functioning?