To my surprise and confusion, this new account suddenly had its "Contacts" populated!
These contacts were correctly related to the email-address I've used for this new account. That email address was (of course) the same as for my old one, on the previous server.
I don't know yet why and how these contacts were automatically created (imported/synchronized?), but the Address book name gives a hint:
This looks like an on-purpose-convenience-feature, but I'd still like to know when and where my data gets exported/imported, so I'll investigate this further...z-app-generated--contactsinteraction--recent
to be continued...
[Related findings]
It seems the active component that created these entries is the:
"Contacts Interaction" Nextcloud App - which is enabled by default.
However, I cannot find any proper documentation and given the "pattern" of these "Recently contacted" contacts, I still don't know where they were imported from. Most importantly: How did this nextcloud-app get access to this data?