Does your test user have a phone activated for Duo Mobile pushes? If you disallow everything but Duo Push, and a given user doesn't have a device activated for Duo Push, the traditional Duo Prompt will offer bypass code entry as an option. The Universal Prompt will always offer bypass codes as an option, whether the user actually has a bypass code available or not.
There is no way to completely prevent use of bypass codes as an authentication method today (like in the authentication methods policy).
Feel free to contact your Duo Care team, your Duo/Cisco account team, or Duo Support to submit a feature request for either removing the bypass code option from Universal Prompt or adding bypass codes as an option to manage in authentication methods policy.
Also, keep in mind that the traditional Duo Prompt reached end of support back in March 2024. If you are still using the traditional Duo Prompt you should be planning your migration to a supported solution!
Duo, not DUO.