I don't know why this would be but I get different responses from your different NS
dig TXT _acme-challenge.faultline.com @ns1.faultline.com +short
"Y8-QfJXqlE2_VG_xlhoco55e60JneJRqZ9ujGEfXbIU"
dig TXT _acme-challenge.faultline.com @ns2.faultline.com +short
"Y8-QfJXqlE2_VG_xlhoco55e60JneJRqZ9ujGEfXbIU"
dig TXT _acme-challenge.faultline.com @ns3.faultline.com +short
"E1T480cPTm56nzx2mlDIQNa-mC7xYlqRzbqyBumO7Os"
dig TXT _acme-challenge.faultline.com @ns4.faultline.com +short
"E1T480cPTm56nzx2mlDIQNa-mC7xYlqRzbqyBumO7Os"
I don't know DNS as well as some of the others but I know that they should all respond the same. Let's Encrypt servers will check any of the authoritative servers. From the error it looks like it checked ns3 or ns4.