This is a regression that was introduced in
This Java 3.x code replicates:
This is a Sub-Document insertion (ADD flag) that's only touching XATTRs, it doesn't specify the body and leaves it at whatever the server defaults to.
It passes on 6.5.0-4908 but fails on 6.6.0-7857, on the last line.
So memcached used to create an empty JSON object as the body if none was specified, and now creates the document with a null body instead.
(This change does seem like an improvement to me, but sadly will break existing released transactional code when running on 6.6.)