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

Can confirm, relates to MC-271402.

Steps to Reproduce:

1. Summon a spider with no AI using /summon minecraft:spider ~ ~ ~ {NoAI:1b}.

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

3. Get the data of the spider using /data get entity @e[type=minecraft:spider,limit=1,sort=nearest].

4. Look for amplifier in active_effects.

Observed Results:
amplifier does not appear.

Expected Results:
amplifier: 3b appears.

Can confirm this too in my own data pack.