As of SERVER-43720, the ReadWriteConcernDefaults cache reuses a command's operation context to look up the latest defaults from config.settings. This read currently does not modify the read concern or read source of the operation context, but should instead always read from the latest snapshot to see the most recent defaults. This can possibly be done by using a ReadSourceScope object (introduced by SERVER-40053 and used when fetching a transaction's history) or an AlternativeClientRegion.