Shurikens don't level when fired from mechanical users (Exu2, 1.12.2), unlike cleavers (when i tested).
How to reproduce:

1. Make shuriken (I used reinforced pink/paper/wood/manullyn)
2. put in mechanical user set to right click
3. have it kill a zombie
4. check exp

The fake player might not be registered with the world, which breaks player tracking. Anyway, this is not something that's supported by design, more by happenstance.