This issue is because, by default, the UDF "AvaTax ship to type" within the document is set to "Line Ship-to Address" and it will search and confirm the address existing in the BP Master Data.
If an address is manually created/modified in a document, then, is required to change the UDF within the document to "Doc Ship-to Address"
If you want to change the default "AvaTax Ship to Type" value permanently, you should go to: Tools - Customization Tools - User Define Fields. And change it there.
If the Document Value does not exist, you can create it with the "New" button