The 1.21.30.21 Preview changelog states:

• When an item is enchanted, their rarity status is now increased depending on their base rarity
  • If the base rarity of an item is Common or Uncommon, it will become Rare when enchanted
  • If the base rarity of an item is Rare, it will become Epic when enchanted
  • If the base rarity of an item is Epic, it will stay Epic when enchanted

Steps to reproduce:

1. Grab any head/skull besides the Dragon Head
2. While holding it, run /enchant @s vanishing
3. Look at its tooltip

Observed result:
Enchanted heads and skeleton skulls stay at the Uncommon rarity.
Enchanted wither skeleton skulls stay at the Rare rarity.

Expected result:
Enchanted heads and skeleton skulls are upgraded to the Rare rarity.
Enchanted wither skeleton skulls are upgraded to the Epic rarity.