A problem that has occurred on a replica set twice thus far - once in production, once during testing. For some reason, sometimes the logical session cache stops refreshing against the main session store, collecting a backlog of logical sessions until it hits the 1 million session cap, which causes queries to start to fail with a 261: TooManyLogicalSessions error.

The tests we did to try and pinpoint the cause of the bug (and with which we caused it again) were using the NodeJS 3.3.2 driver (for Node 10.16.3), and the Python 3.9.0 driver (for Python 3.7.4)  respectively.

The test environment was a 4.0.12 replicaset with one primary, one secondary and an arbiter.