Certain entities store an "Item" value in their NBT saving. This field is not registered with the entity in the DataConverter Schemas. This issue has probably not manifested itself yet due to the unlikeliness of these entities to save and exist for conversion. Thus, this issue report is an fyi to something that should be fixed for completeness sake in the future.
Specifically. In the schema numbered 1460. The following entities are lacking an "Item" field registration.
minecraft:eye_of_ender_signal (since renamed to minecraft:eye_of_ender)
minecraft:xp_bottle (since renamed to minecraft:experience_bottle)
minecraft:snowball
minecraft:ender_pearl
minecraft:egg
minecraft:small_fireball
I don't know if these fields are even used by the entity. Just making a code observation.
Comments 3
I can confirm this issue in Minecraft 1.21 pre-release 1. It affects the componentization, so this issue can be reproduced without editing the NBT of an entity.
The entity's item stack will always reset to the default stack of the given entity's default item as a fallback, as the item stack format itself has changed.
Steps to reproduce
1. Create a world in Minecraft 1.20.4
2. Create a bubble column using soul sand and water
3. Obtain an enchanted snowball item using creative mode and an anvil
4. Throw the snowball into the bubble column vertically so that it stays in stasis
5. Observe that the snowball visibly has an enchantment glint
6. Load this world in Minecraft 1.20.5 or later
7. Observe that the snowball no longer has an enchantment glint
Note that the use of a bubble column is optional, but helps with reproducing as otherwise, the snowball would move and disappear quite quickly.
The command /data get entity @e[type=snowball,limit=1,sort=nearest] Item shows the new NBT of the entity's item stack.
Thank you for your report!
After consideration, the issue is being closed as Won't Fix.
Please note that this is not the same as Working as Intended, as this bug report correctly describes behavior in the game that might not be the intended or desirable behavior, but it will not be fixed right now. Sometimes, this is because the issue reported is minor and/or impossible to change without large architectural changes to the code base.
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I should add on. This issue would include older registrations such as in Schema version 99 or 705. But I am not sure when the "Item" field was added. So it may not go back that far.