The Archiving part of the Monitoring plugin stores chat logs. These logs can be retrieved again by the clients.
To improve performance, logs are temporarily cached, and written to a database periodically.
When a client retrieves logs, those that have not (yet) been written to the database are not included in the result.
The intended behavior is that all logs are retrieved, not only those that have been written to the database, but also those that are buffered, waiting to be written to the database.
The impact of this issue is reduced greatly by the fix for OF-1487, although without further fixes, gaps still can occur.
Fixed in Monitoring plugin version 1.6.0.