SDK3: subdoc error "only a single xattr key may be accessed at the time" creates an UNKNOWN error

JsonObject obj = JsonObject.create()

        .put("v", v)

        .put("u", clientUuid)

        .put("t", LocalDateTime.now().toString());

return collection.reactive().mutateIn(PERSISTED_LOG_DOC_ID,

        Arrays.asList(

                MutateInSpec.increment("count", 1).xattr().createPath(),

                MutateInSpec.arrayAppend("logs", obj),

                MutateInSpec.upsert("logs[-1].c", "${Mutation.CAS}").xattr().expandMacro()),

        MutateInOptions.mutateInOptions().upsertDocument(true))