Testing discovered MC-270988 - I reported that one first while testing this.
The bug
Spawn eggs from versions before 1.12.2 will obtain an extra component if upgraded to 1.20.5. This component is redundant as it only encodes the target entity ID, which is already handled by the spawn egg item ID itself.
This was originally reported under MC-257688 and obtained Won't Fix status, but since NBT has been replaced entirely by components the upgrade process is likely different now.
How to reproduce
Obtain spawn eggs in 1.12.2
Upgrade this world directly to 1.20.5
Obtain spawn eggs in 1.20.5
Try to stack them with your 1.12.2 spawn eggs
Expected results
They would stack.
Actual results
They do not.
How to fix
Upgrade spawn eggs with only the standard NBT from the creative inventory (or the iron golem/snow golem/ender dragon/wither eggs) without assigning the extra component. If it has any other data, keep the NBT/components.
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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