Altering the location of Oracle-Suggested Backup
I cannot say for sure why RMAN behaves in this way, but as a workaround you can edit the resulting script within the same workflow in EM before the job is created:

 ALLOCATE CHANNEL disk1 DEVICE TYPE DISK FORMAT '/disk1/%d_backups/%U';


This will ensure the backups are placed in the destination you specified regardless of RMAN settings in control file saved with CONFIGURE command.