Please review 101: reverse proxy to understand mechanics used by a reverse proxy integration. Note that this only required in case reverse proxy is in place (or similar like cloudflare).
this might be OK as long your system reflect the right CODE URL ind admin settings (https://cloud.mydomain.tld/settings/admin/richdocuments):
BTW is this the same problem discussed in Help with Unauthorised WOPI host. (TrueNas Install + Remote Collabora Server) ? please avoid additional load on volunteer helpers with duplicate posts!