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:
Give yourself a piece of gunpowder, any dye and any mob head.
Either open the survival inventory or place down a crafting table and open its GUI.
Place the gunpowder, the dye and the mob head in the crafting slots.
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.
Can confirm: Relates to MC-275598 MC-275487 MC-275485 and MCPE-184817