Did you also read this section? https://docs.nextcloud.com/server/latest/user_manual/en/files/access_webdav.html#id10
I don’t use it my self, but afaik, they introduced “per-app selection of IP addresses” with the release of TrueNAS 25.04 (Fangtooth): 25.04 (Fangtooth) Version Notes | TrueNAS Documentation Hub
Could possibly be solved with a reverse proxy and a proper split-brain DNS setup.