The logical session cache reaper thread will initially set up the sessions collection. As a part of this, it runs createIndexes which takes a database exclusive lock. Currently, if it runs before the replication system has been set up (and its config initialized), it will wait an entire refresh interval before setting up the collection. This can cause the setup to occur at some later point while the database is up and running, and has the potential to cause transaction aborts due to lock timeouts.