Thank you for posting here in the Community. I know a way how you can stop auto-assigning categories when downloading bank transactions in QuickBooks Online ( QBO).
You can edit the bank rule that you've created and then turn off Auto-add settings. This will stop the system from automatically assigning categories. I'll show you how:
Choose the Banking menu and go to the Rules tab.
Find the rule that you want the Auto-add feature to get rid of and tap on the Edit link below the Actions column.
On the Edit rule window that appears, scroll down and click the button under the Automatically confirm transactions this rule applies to the option.