[knot-resolver-users] Resolving problem connectivity.samsung.com.cn even on dual-stack

Hello everyone, at AS50242 we experience problem with resolving connectivity.samsung.com.cn We run two servers, each with 4 instances. Both servers have working dual-stack (v4/v6). knot-dnsutils/unknown,now 3.1.1-cznic.1 amd64 [installed] knot-resolver-module-http/unknown,now 5.5.0-cznic.1 all [installed,automatic] knot-resolver-release/unknown,now 1.9-1 all [installed] knot-resolver/unknown,now 5.5.0-cznic.1 amd64 [installed] Dnsviz shows problem reaching few IPv6 servers of .cn TLD via UDP. I can not understand, why both of our servers response with SERVFAIL. Any ideas how to troubleshoot more? Thank you, Blažej