Thanks, that helps. Some VMs fail to the Synology but pass to S3, and some the other way round.
So I think the backups on Ark are probably fine (they pass health checks, as you said at the start) and it has more to do with what each target already holds, but I could be wrong.
I'd also flag the several delta jobs with different retentions all feeding Ark, I just don't know the mirror code well enough to say whether that matters.
@Team-XO-Backend, posts 6 and 10 have the log and the pattern if someone gets to this one.