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

1. Obtain spawn eggs in 1.12.2
2. Upgrade this world directly to 1.20.5
3. Obtain spawn eggs in 1.20.5
4. 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.