If a session is used for a transaction that commits at a timestamp T, then that session can never be used for a transaction with an atClusterTime value <=T because it will trigger a conflict when updating the session document in the config.transactions collection, even if there are no conflicts with documents in user-space. See here for a demonstration of the issue in mongosh: https://gist.github.com/kmorkos/8b9bb5493ec0db3f64e711ae02cc0311

For background on why this change would be useful: the current behavior makes it impossible to use arbitrary atClusterTime values for transactions in application code that uses a driver since the driver's session pooling makes it non-deterministic if a recycled session was used for a transaction after the given timestamp or not. IOW the current behavior violates the "statelessness" property of sessions that session pooling assumes.