Garry Bargsley covers the behavior of a trace flag:
In SQL Server environments where transactional replication runs alongside Always On Availability Groups (AGs), DBAs sometimes face a frustrating scenario: replication stalls when a secondary replica or subscriber is offline for maintenance, patching, or unexpected downtime.
By default, SQL Server’s Log Reader Agent is cautious. It only marks transactions as ready for replication once they are hardened on both the primary and all replicas. This ensures consistency across the AG, but it can also cause replication to stall if an asynchronous replica or subscriber is unavailable for an extended period of time.
Click through to see how it works and what the consequences are.