Issue
A Review App is being recreated on every change to the related Pull Request instead of simply being re-deployed with the new code.
Resolution
It's likely your Review App's Release Phase task has not successfully completed. Because of this, changes to the Pull Request will result in the deletion and recreation of your Review App.
To confirm this, check the status of the Release Phase:
$ heroku releases --app review-app-pr-1
v4 Deploy 43459d4f release command failed
If you're having trouble determining why your Release Phase is failing, you can view it's output:
heroku releases:output --app review-app-pr-1
For further debugging, try removing the Release Phase from your Procfile and redeploying the app. Then run the command in a one-off dyno:
heroku run my-release-phase-script.sh --app review-app-pr-1