Yes, this is intended to work that way. Remember, it's the legacy front-end backup method. Full support for warning and error reporting is only possible when you're using the remote JSON API for taking backups.
The legacy front-end backup method replies only on redirections. The final redirection leads to a page that returns HTTP 200 OK unless there is a PHP or server error. This allows you to check whether your server completed the backup process. The body of this last page load is plain text, begins with a three digit numeric code followed by a space and some text. The three digit code is 200 if the backup completes successfully or 500 if a catchable backup time error occurs.
That said, the best thing you can do is setup another CRON job for the failed backup checks (please consult the Scheduling Information page in Akeeba Backup on your site). This must run after the expected finish time of the backup. It checks the database information to find out if you have a stuck or failed backup. In this case it will email you, telling you that the backup has failed. If the backup is successful no email is sent. This operation is designed to deal with use cases like yours.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!