According to the 24w33a changelog:

Any item or block which can be crafted must inherit the highest rarity of its crafting ingredients.

Firework stars crafted with a mob head are common, despite the mob head's rarity being higher.

How to reproduce:

1. Give yourself a piece of gunpowder, any dye and any mob head.
2. Either open the survival inventory or place down a crafting table and open its GUI.
3. Place the gunpowder, the dye and the mob head in the crafting slots.
4. Look at the crafting output.

Expected result:

The firework star resulting from the crafting recipe would be uncommon, rare or epic (depending on the mob head used), since that's the highest rarity of its ingredients.

Observed result:

The firework star resulting from the crafting recipe is common, despite the mob head used as an ingredient.