Weapon with Bane of Arthropods can only give the target slowness I instead of slowness IV

Steps to Reproduce:

1. Summon a spider with no AI.

   /summon minecraft:spider ~ ~ ~ \ {NoAI:1b}

2. Attack the spider with a weapon enchanted with Bane of Arthropods.
3. Get the data of the spider.

   /data get entity @e[type=minecraft:spider,limit=1,sort=nearest] active_effects

4. Look for amplifier in active_effects.

Observed Results:
amplifier does not appear.

Expected Results:
amplifier: 3b appears.