There are cases where the title ownership has to be transferred with CrossRef, even if AUP is already the owner of the title.
This may happen when a title is published on a third party online platform, and that the platform registers the DOIs for the title under the name of the platform owner. In that case, CrossRef registers the platform owner as the owner of the title, even if it is really AUP (or anyone else).
So in those cases, too, the ownership needs to be transferred with CrossRef.
Don’t ask me how we know this 🙄