__Host-Prefix in subdir

I’m using linuxserver/nextcloud and linuxserver/letsencrypt. I have it setup as subdir so the URL to nextcloud looks like this https://mydomain.com/nextcloud. I can access it just right, so no issues but I get “A” rating on https://scan.nextcloud.com/

From what I’ve read only setup as subdomain it can get rid of the host prefix issue. Is this true?

Indeed: Security Scan: __Host-Prefix
mydomain.com would also work, but Nextcloud must be accessible on webroot, not subdir.