I did the migration asked from Google because Fabric will die next year. But the crashes reports still are available in Fabric. They are available in both tools: Firebase and Fabric.
Looking the Docs, they still asks you to include Fabric library in your app (https://firebase.google.com/docs/crashlytics/get-started?platform=ios)
So, it's something wrong in my migration?
Firebase uses the same Fabric Crashlytics SDK to calculate the crashes on both Fabric and Firebase dashboards. After March 31st 2020 Fabric Crashlytics dashboard will deprecate and developer use only Firebase Crashlytics for their crash information.