Why Incremental Backup Size Doesn’t Change Much Between Level 0 and Level 1 in Oracle

Yes, if you want to prevent a level 1 from being created without a corresponding level 0 being available, then you need to do a crosscheck in the script so RMAN knows that the level 0 is not available.

Prior to 10g (or with compatibility < 10.0) Oracle would do a level 0 when a level 1 was done without a level 0. Since you are on 11g, I would expect your compatibility to be >=10.0, so RMAN "copies all blocks that have been changed since the file was created". This behavior seems to have changed between versions and there is some confusion as to what the current behavior is, but your findings are consistent with the documentation.

The RMAN backup concepts guide (link for 11gR2, but the concept hasn't changed):

The only difference between a level 0 incremental backup and a full backup is that a full backup is never included in an incremental strategy.

Full and level zero backups copy all blocks that have data (and more if you're doing image copies), so they are both "full" in that sense.
But if you want to do cumulative or differential backups, the starting point for those must be a level zero incremental backup.