If all transactions are importing using the same payee name in Banktivity for Mac, you probably have an Import Rule that is applying this payee name to all transactions when importing.
Click on the word Configuration in the bottom left corner of the document, then choose Payees, Schedules & Rules from the menu that appears. When the screen loads, click on Import Rules at the top of the screen to view your list of Import Rules. Look for a rule with the Payee name, Category, or Note which is being applied to your importing transactions erroneously and deleting the existing Import Rule for this Payee name.

For more details on Import Rules you can also refer to the section on import rules in the Banktivity Help Files: https://www.banktivity.com/help/v9/importing/rules.php