The bug

From 11w49a (1.1) to 17w46a, spawn eggs existed for the snow golem, iron golem, ender dragon and wither. 17w47a (1.13) changed spawn eggs so that each egg is its own namespaced ID, rather than one centralized item ID (spawn_egg) with its target entity specified via NBT. As such, upgrading a world containing a spawn egg from a pre-1.13 world into a world in 1.13 or a newer version will convert it into its corresponding spawn egg item ID if a matching one is found. Spawn eggs for other entities are instead changed into pig spawn eggs with NBT that preserves the initially spawned mob (see also MC-124065).

22w44a re-introduces proper spawn eggs for the wither, ender dragon, iron golem and snow golem. As these spawn eggs existed prior to 1.13, it'd be expected that worlds containing them would have them upgrade to these new spawn egg types, but they do not.

How to reproduce

A 1.12.2 world containing a snow golem spawn egg, iron golem spawn egg, wither spawn egg and ender dragon spawn egg in a chest directly in front of the player is attached.

1. Download the attached world
2. Open this world in version 1.12.2
3. Mouse over the four gray spawn eggs, confirming the entity that they spawn (you can also give yourself a spawner and use the eggs on this spawner to confirm they do indeed contain the mob in question)
4. Open the world in snapshot 22w44a or (if this isn't fixed immediately, which hopefully isn't the case) a later version
5. Check on the four spawn eggs
6. Use each one to confirm its identity

Expected results

The four spawn eggs from 1.12.2 would be converted to their 22w44a+ counterparts accordingly.

Actual results

These four spawn eggs instead become pig spawn eggs with extra NBT.

How to fix

A data fixer should be implemented that points these four old spawn eggs towards the new alphanumeric IDs so they convert in the exact same way as all other pre-1.13 spawn eggs.

Further notes

The NBT on these pig spawn eggs mean that they function identically to the 22w44a spawn eggs, but are very hard to distinguish from each other without directly using them (MC-249697), which for the ender dragon and wither spawn eggs is VERY dangerous.