RMAN controlfile retention question

 Can someone please comment on our issue here.  We are new to RMAN, have been using scripts for years.  Our newest database, we are trying to implement RMAN. This script is simple (see below).  Our issue concerns the controlfile backup.  With controlfile autobackup off, we know RMAN includes the controlfiles and spfile in one of the datafile backups.  Then we do the explicit backup current controlfile further down the script.  We then ran the backup 3 times.  After the 3rd backup, we see that the controlfile backup for the oldest backup is now gone.   Per the SR we have open, since RMAN is automatically