binary log rotation not honouring expire_log_days

Binary logs are not being deleted by mariadb server in line with the expire_log_days variable. I have been watching this for a while on 10.2.7 and it seems random when deletion actually takes place (might it possibly be being triggered by some specific action occurring, other than time passing?).
Currently I have expire_log_days = 2
as can be seen from the directory listing attached, the logs go back to 16:11 on 31-Jul so however it is initiated (e.g. (1)run once at midnight, (2) check every minute or hour), the logs from 31-Jul and 01-Aug should have been removed.
This has a potential impact regarding available space,and whilst it shouldn't rely on narrow margins, it should be able to be accounted for (assuming consistent usage!)