Running hybrid situational tests against 6.0.3 and 6.5 (and I expect with 5.5.6 - tests pending) with LCB commit a20cb22301e2e5d9d7d12dba14e652da436a2618 (2.10.6 candidate) is reporting LCB_KEY_ENOENT for all KV ops about 50% of the time. The other 50% reporting success. As seen in this graph report http://sdk-testresults.couchbase.com.s3.amazonaws.com/SDK-SDK/CB-6.0.3-xxxx/RSTConnect-HYBRID/02-20-20/043245/67601693ebdb7400078222fc804c1d28-MC.html
I've attached the logs for the test above. But the result is not unique to this test and occurs in all of the KV parts of the hybrid tests. If any other logs are wanted they can be found here for 6.0.3 http://sdkbuilds.sc.couchbase.com/view/LCB/job/sdk-lcb-situational/job/sdk-lcb-situational-release/job/c-sdk-alice/lastCompletedBuild/testReport/ and here for 6.5 http://sdkbuilds.sc.couchbase.com/view/LCB/job/server-build-test-lcb/job/centos-lcb-sdk-server-situational-tests/lastCompletedBuild/testReport/ Just click on the particular test and downloading the SDK log.
Could be related to another older issue that was filed CCBC-1013
Full spreadsheet of results found on the linked SDKQE-1851.