Oracle RMAN - MAXPIECESIZE ignored for Level 0 backups
Try removing the MAXPIECESIZE from the configuration of the channel, and use SECTION SIZE 10G on the backup command.  The following is working for me:

run {
        backup section size 10g incremental level 0  as compressed backupset archivelog all database;
        delete noprompt obsolete;
}


You may need to set paralellism to > 1:

CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO COMPRESSED BACKUPSET PARALLELISM 2;


If that does not solve the problem, please post the output of show all;