Invalid format specified for SUBDOC_MULTI_MUTATION

We've started seeing this error in eventing log files since last week, which appears to coincide with lcb revision to 2.9.0:

2018-06-04T20:26:30.265330Z INFO 505: HELO [{"a":"libcouchbase/2.9.0_2_g4224062 (Linux-3.13.0-96-generic; x86_64; GNU 7.3.0)","i":"000000003bbbe475/805f7788cd0070ea"}] TCP nodelay, XATTR, XERROR, Select bucket, Snappy, JSON, Tracing [ 172.23.104.17:39382 - 172.23.104.16:11210 (not authenticated) ]

2018-06-04T20:26:30.266842Z INFO 505: Client 172.23.104.17:39382 authenticated as <ud>@eventing</ud>

2018-06-04T20:26:30.271437Z WARNING 490: Invalid format specified for SUBDOC_MULTI_MUTATION - 4 - closing connection packet:mcbp::header: magic:0x80, opcode:0xd1, keylen:20, extlen:1, datatype:0x0, specific:64, bodylen:1138, opaque:0xf71733, rawextras:1

It is not clear to me where the error is, and if eventing as a user of libcouchbase can do anything to cause the invalid format error.

Requesting for help from SDK team for further investigation.

The logs below, originally from MB-29969 shows the issue:
s3://cb-customers-secure/mb-29969-01/collectinfo-2018-06-05t040523-ns_1@172.23.104.16.zip
s3://cb-customers-secure/mb-29969-01/collectinfo-2018-06-05t040523-ns_1@172.23.104.17.zip
s3://cb-customers-secure/mb-29969-01/collectinfo-2018-06-05t040523-ns_1@172.23.104.18.zip
s3://cb-customers-secure/mb-29969-01/collectinfo-2018-06-05t040523-ns_1@172.23.104.19.zip
s3://cb-customers-secure/mb-29969-01/collectinfo-2018-06-05t040523-ns_1@172.23.104.21.zip
s3://cb-customers-secure/mb-29969-01/collectinfo-2018-06-05t040523-ns_1@172.23.104.23.zip
s3://cb-customers-secure/mb-29969-01/collectinfo-2018-06-05t040523-ns_1@172.23.104.25.zip
s3://cb-customers-secure/mb-29969-01/collectinfo-2018-06-05t040523-ns_1@172.23.104.93.zip