You should get a PM with the log output if it fails. It’s sent either directly to you if it’s a manual backup in the UI or to the admins group if it is an automatic backup.
An exception during the backup should also show up in /logs and, I think, in one of the log files as well. Try searching for EXCEPTION:
But, the fact that it keeps temporary files around makes me wonder if Sidekiq or even Docker or the host get restarted during the backup. That would explain why the cleanup doesn’t run and why you aren’t getting a PM.