Before a 3.6 internal client has issued its initial isMaster command, its SessionHandle is protected from being terminated by the kPending tag. The isMaster command removes the kPending status but (when the client is new enough) adds kLatestVersionInternalClientKeepOpen status, which protects the session from being terminated by a featureCompatibilityVersion upgrade.

However, there is a small window between the unset and the set during which there is nothing to stop a featureCompatibilityVersion upgrade from terminating the session, which is causing failed updates and inserts in the toggle_feature_compatibility.js test.