Your IPv4 and IPv6 IPs do NOT return the same HTTP content:
curl -Iki4 http://upczone.cz/
HTTP/1.1 301 Moved Permanently
Server: nginx
Date: Wed, 29 Jan 2020 03:02:30 GMT
Content-Type: text/html
Content-Length: 178
Connection: keep-alive
Location: https://www.upczone.cz/
X-Content-Type-Options: nosniff
Referrer-Policy: strict-origin-when-cross-origin
curl -Iki6 http://upczone.cz/
curl: (52) Empty reply from server