I can't share the domain name but its DNS servers are:
ns3.cisws.nl
ns6.cis-websolutions.nl ns3.cisws.nl
ns6.cis-websolutions.nlPicking a random domain hosted on those nameservers, mdfs.net, it looks like, of the 4 IPs, 2 are down and 1 of the remaining ones doesn't support TCP.
http://dnsviz.net/d/mdfs.net/W48OcQ/dnssec/ https://ednscomp.isc.org/ednscomp/4040283963
1.1.1.1 is less tolerant than some resolvers of that level of breakage.
https://community.cloudflare.com/t/ipv6-timeouts-appear-to-b...
I'm not sure nothing else is wrong, but the IPv6 issue is likely why 1.1.1.1 is having trouble resolving it.