This host 95.143.172.147 offers a working http answer (200) for dns-http500.klml.org

There are two DNS entries for Round-robin_DNS
The other host 95.143.172.223 will give you a http status 500 (for dns-http500.klml.org).

curl is doing no failover:

curl -vI http://dns-http500.klml.org
* Rebuilt URL to: http://dns-http500.klml.org/
*   Trying 95.143.172.223...
* TCP_NODELAY set
* Connected to dns-http500.klml.org (95.143.172.223) port 80 (#0)
> HEAD / HTTP/1.1
> Host: dns-http500.klml.org
> User-Agent: curl/7.58.0
> Accept: */*
> 
< HTTP/1.1 500 Internal Server Error
HTTP/1.1 500 Internal Server Error
< Date: Thu, 01 Nov 2018 17:03:15 GMT
Date: Thu, 01 Nov 2018 17:03:15 GMT
....

Try also dns-no-tcp.klml.org with two DNS entries with one without any TCP/IP answer