This could be because the app wasn't permanently deleted yet
"When you remove an app from your Firebase project, the app is scheduled to be automatically permanently deleted after 30 days. During that 30 days, you can restore the removed app before it's permanently deleted. After the permanent deletion, though, you cannot restore the app.
Therefore, before removing an app from your Firebase project, make sure that permanent deletion of the app from your project is what you really intend."
"How to immediately and permanently delete an app from a Firebase project
If you need to immediately delete an app from a Firebase project, you can do so anytime before Firebase automatically deletes it (which happens 30 days after it's been removed). If you immediately delete an app, the app will be permanently deleted and cannot be restored.
Note that you have to remove the app from your project first and then perform an additional set of actions to delete it immediately.
Follow the instructions above to Remove an app. After removing the app, go back to the Your apps card, and then click Apps pending deletion. In the row for the app that you want to delete immediately, click DELETE NOW. Confirm the changes that will occur with the permanent deletion of the app. Click Delete app permanently."