Backup requesting archivelog

We recently replaced the mount where our archivelogs were sitting and some of the older logs were lost in the transition. I'm getting them restored now. Unfortunately, a database backup had not been run since March 20th, an oversight that I'm trying to fix.

Question: If we're doing a full backup (we don't do incremental level 1 backups for this database), why would it fail because it couldn't find one of the older archivelogs? I haven't found anything in my searches to explain why this would occur. Another DBA on the team is asking me for an explanation and I don't have the answers.

Thank you for your input.