Upgrading from 1.12.2 and below will cause items to lose enchantments

The bug

When a world is upgraded to 20w18a from 1.12.2 or lower, items in your inventory and ender chest will lose their enchantments.

How to reproduce

1. Start a world in 1.12.2 or lower

2. Put items with enchantments in your inventory and an ender chest

3. Loaded the world in 21w18a.
   → ❌ The enchantments were wiped

Upgrading to 1.16.5 first will keep the enchantments.