DNS exfiltration covert channel detected

Your network-monitoring dashboard shows one internal app server generating an unusually high volume of DNS queries, 8,000/min, up from ~50/min baseline, almost all TXT and long-label A-record lookups to subdomains of a single odd-looking domain (e.g., a1b2c3d4.exfil-cdn[.]xyz). Outbound HTTP/HTTPS egress from that host looks normal and low. The host runs a payments microservice. A dependency was updated in last week's deploy. Conventional DLP (which watches HTTP) saw nothing. How do you triage, contain, and remediate this suspected covert channel?