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. 
 Basics - Manage team access - AWS Amplify Docs
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.
Dead AWS Amplify Apps can't be deleted?asked 9 months ago
Amplify configured for next.js is not removing all resourcesasked 4 months ago
[BUG] Amplify Console - Previews not workingasked 6 months ago
Amplify auto generated resourcesasked 5 months ago
Amplify does not clean up after app deletionAccepted Answerasked 2 months ago
Amplify Build Times with React SPAasked 10 months ago
Amplify headless - need help on setting up the app automatedasked a year ago
Amplify and React App: Access Deniedasked 5 months ago
View/modify amplify CLI published apps in the AWS console?asked 3 years ago
Amplify Studio not accessible for created IAMasked 24 days ago