You want to know why the 'Service Number', 'Site Number' or 'Customer Location Identifier' is being removed when saving an invoice in QuickBooks.
This is expected behavior and happens when "Enable address validation before tax calculation" setting is turned on.
When this setting is enabled, AvaTax validates the "Ship To ( Service # for this customer )" address and updates the address returned by Avalara address service into "Ship To" field. This address is updated only for that particular transaction and not for the customer. When we (Avalara) update this, QuickBooks itself makes the dropdown field blank. This is the default behavior of QuickBooks and it blanks out the drop down field even if AvaTax is not present and the address is changed at transaction level.
If you wish to keep the 'Service #' disable (uncheck) "Enable address validation before tax calculation" setting from AvaTax configuration.