Change how the level is stored.

Before, you can get high level enchantment if you just rename a mob, for example..

If a Level 1 Zombie spawn and you rename it to ZOMBIE 1000 using nametag, it will drop an enchantment that has a level of 1000 instead of level 1.

Now, even if you rename it to any number, their level when they spawn will persist. A level 25 zombie will always drop a level 25 enchantment even if they got renamed.