lastReadSeqno Monotonic exception "::lastReadSeqno) invariant failed:"

Exception seen as follows

 CRITICAL Caught unhandled std::exception-derived exception. what(): Monotonic<m> (ActiveStream(eq_dcpq:"i":"AE23217F111AEFB7/0528E92BBF8F1215","a":"kafka-connector/4.1.13 (blah) java-dcp-client/0.47.0 Java/11.0.20 (Azul Systems, Inc.; OpenJDK 64-Bit Server VM; 11.0.20+8-LTS)" vb:121)::lastReadSeqno) invariant failed: new value (555) breaks invariant on current value (555)

And this was generated from the assignment in ::snapshot

• https://src.couchbase.org/source/xref/7.2.4/kv_engine/engines/ep/src/dcp/active_stream.cc?r=a5eef6ee#1495

This was only ever observed on a Kafka DCP connection, and seen twice. Analysis strongly suggests that at the time Kafka was connected to a replica vbucket.

Possibly this was a vbucket that rolled back (the original issue occurred outside of memcached.log coverage), but yet to reproduce in unit tests anything similar. Timestamps from ns_server shows the exception occurred some minutes after vbucket became replica, so maybe not rollback related.

No further insights to the cause known.