Firstly I'm only a Junior/mid level DBA so I could be wrong with this.
Secondly I appreciate it's a copy-only backup so perhaps not part of your normal backup strategy.
Your questions make me wonder whether you test your regular backups. My understanding (and remember I'm only a junior DBA) is that it's advisable to do this quite regularly. It's possible to get caught in "SQL server/SQL agent says the backups complete so it's everything is fine" when actually there is an issue. SQL agent server/agent isn't being inaccurate it just doesn't cover all scenairos.
Apologies if I'm being patronising or if I've missed the point. Also apologises for going off tangent to your question.