Sorry for a long delay in our response. We did some additional tests and checked email alerting for backup MP once again and we can confirm that there is nothing special about Alerts in Veeam Backup MP, so your regular notification rule should be able to deliver the corresponding alert. Do you actually see Backup objects in the console? Do you see the alert itself?
Also, could you please send us the screenshot of your subscription configuration with the Criteria description? We would highly appreciate any kind of additional information which could help us to investigate the issue you are experiencing.
Also a weird thought, but could it be that it's in the Junk mail blocked because of some words combination in the alert text or name of the backup job?