Likely a web-server misconfiguration (check out name-based virtual hosting[0]). Adding a trailing dot after the TLD (this signifies the root of the DNS naming hierarchy) works fine, and you're referring now to an absolute name (FQDN). How this get handled afterwards at the webserver side, depends not just on the resolved IP, but on the name itself (which is passed from client to server in the HTTP Host: header). And that's where this might fall apart if the administrator didn't provide for this situation.

[0] https://httpd.apache.org/docs/current/vhosts/name-based.html