Piglin cannot be spawned/edited to hold an item in their offhand unless their main hand is empty

Summon a piglin with any secondary item in their offhand and the item is stored in their inventory or placed in an armor slot rather than held in their offhand, unless their main hand is also empty. If the offhand item is golden this will take the place of the primary held item (e.g. golden sword will replace the crossbow) or will be worn (e.g. golden leggings will be put on instantly).

Example:

1. /summon piglin ~ ~ ~ {equipment:{mainhand:{id:"crossbow"},offhand:{id:"firework_rocket"}}}
   The piglin does not have a firework rocket in its offhand

2. /data get entity @n[type=minecraft:piglin] equipment
   The firework rocket is indeed not in its offhand

3. /data get entity @n[type=minecraft:piglin] Inventory
   It’s in its inventory instead