Amplify does not clean up after app deletion

Is this normal behaviour

Yes, it is normal.

is there a reason why it does that?

The reason for this is unclear, but we assume it is probably due to the split functionality between Amplify Hosting and Amplify Studio.

Cognito user pools beginning with amplify_backend_manager and Lambda functions with the following names are automatically created when Amplify Studio is activated. [1]

・amplify-login-create-auth-challenge-SHORT_CODE
・amplify-login-custom-message-SHORT_CODE
・amplify-login-define-auth-challenge-SHORT_CODE
・amplify-login-verify-auth-challenge-SHORT_CODE

[1] Basics - Manage team access - AWS Amplify Docs
https://docs.amplify.aws/console/adminui/access-management/

The above resources will be automatically removed by disabling Amplify Studio before removing the Amplify app.

Therefore, if you have been deleting the above resources yourself, we recommend that you disable Amplify Studio before deleting the Amplify application.