Quick update: I just manually ran another system-wide backup and it was successful, so it seems it may have just been a one-off and probably won't happen again anytime soon, was likely just coincidental I was making a bunch of changes to it yesterday considering other system backups proceeded without issue since the time I made those changes. But I'd love to get a second-set of eyes for insight into if this is something I should fix to prevent it in the future or just a one-off type of thing that can safely be ignored.