Aurora MySQL: how do I keep binlogs around after reboot?

We have a request to enable binlogs on an Aurora MySQL instance and to keep those logs around for a certain amount of time. To test, we enabled binlogging with binlog_format=ROW and set our retention time to 1 hour.

call mysql.rds_set_configuration('binlog retention hours', 1);

However, we are seeing inconsistent behavior when we reboot the instance. A new binlog gets created and the old one *sometimes* drops after only a couple of minutes. It was my understanding that setting the retention hours would keep the old file(s) around for an hour. There is also the global config expire_logs_days=0, would that affect keeping the old file(s) around?