trident durability is very low but can be full durability

I try to make very low durability that trident can't use ranger. I put low durability trident in dispenser and i tap lever. When trident hit me , i take trident and it seen full durability.

What should happen:
Trident should be brock
What happened:
Trident become full durability for free

Step:
1_/gamemode s
2_make very low durability trident (/give @p trident 1 300)
3_put very low durability trident in dispenser (screenshot)
4_place lever near dispenser
5_go near the dispenser then tap lever to hit yourself (screenshot)
6_it seen trident not brock. Take trident and look durability trident. It seen full durability(screenshot)