Summary:
The contents slot for the /item command does not function with shot snowballs, eggs, ender pearls, and thrown potions.
Fix MC-270119 was not applied to the above entities.
This has only been fixed for the following entities: minecraft:arrow, minecraft:spectral_arrow, minecraft:trident, minecraft:fireball, minecraft:spectral_arrow, minecraft:small_fireball.

Steps to reproduce:

1. /summon minecraft:snowball ~ ~ ~ {NoGravity:1b}

1. /item replace entity @e[type=minecraft:snowball,limit=1,sort=nearest] contents with minecraft:acacia_boat

Observed results:
The snowball's item field was not substituted with an acacia boat, and an error message was encountered.

Expected results:
The snowball's item field should be replaced with an acacia boat as anticipated.