If the PDB$SEED is not updated then the INCR BACKUP LVL 0 rman does not backup it.
To prevent this and to get each time you perform the INCR BACKUP LVL 0 the content of the datafiles of the PDB$SEED, here is the command to run :
BACKUP INCREMENTAL LEVEL BACKUP_LEVEL DATABASE FORCE FILESPERSET 20 FORMAT '/appli/oracle/backup/%d/%T/%d_lvl_BACKUP_LEVEL_%U.bkp' TAG='LVL_BACKUP_LEVEL_%d_%T';
BACKUP AS BACKUPSET CURRENT CONTROLFILE FORMAT '/appli/oracle/backup/%d/ACCOUNTBACKUPDATE/%d_ctl_lvl_BACKUP_LEVEL_%U.bkp' TAG='CTL_%d_lvl_BACKUP_LEVEL_%T';