Office integration mostly doesn’t “just work” as the protocol is rather complex - see Collabora integration guide for details.
AiO runs a “separate” instance of “own server” CODE - the specialty it is configured to share the domain with Nextcloud - it’s not bad itself but makes it little harder to understand and separate the components for troubleshooting. this instance uses e.g. https://cloud.mydomain.tld/hosting/discovery
and https://cloud.mydomain.tld/hosting/capabilities
to advertise its functionality.
your URL
belongs to a “built-in” CODE variant.. but IMHO there is no good reason to use it at all.
At the end your system (including external parts like reverse proxy) must be configured in a proper way and try and error or mix and match will probably fail.