The bug

When upgrading from 1.15.2 or 20w11a to 20w12a, item attribute modifiers are not datafixed to the new UUID format, rendering them broken.

To reproduce

In 20w11a:

/give @p stone_sword{AttributeModifiers:[{Name:"generic.attackDamage",Operation:0,Amount:50,Slot:"mainhand",UUIDMost:1L,UUIDLeast:2L}]}

Upgrade to 20w12a and check the sword: it no longer has extra attack damage because the NBT wasn't upgraded