Call Recording Reports not deleting zero byte gsm files

I have "Remove zero duration files" set to yes in the GUI. Manually running the cron task:

[root@uc-21359467 ~]# fwconsole  recordingreport -s
Synchronizing files to database
Warning: Deleting zero length files
 10/10 [============================] 100%
Synchronization was done in 9 seconds

So it appears to be purging zero length files, but files like this remain (small sample):

-rw-r--r-- 1 asterisk asterisk       0 Oct  4 08:03 external-453-1630336xxxx-20181004-080300-1538658180.705.gsm
-rw-rw-r-- 1 asterisk asterisk       0 Oct  4 02:25 external-453-1708668xxxx-20181004-022559-1538637959.65818.gsm
-rw-r--r-- 1 asterisk asterisk       0 Oct  4 10:13 external-453-1708867xxxx-20181004-101343-1538666023.2663.gsm
-rw-r--r-- 1 asterisk asterisk       0 Oct  4 10:15 external-453-1708867xxxx-20181004-101511-1538666110.2773.gsm