The bug

The problem is that you are unable to tell from the NBT data whether the SpawnData entity is the same entity object also stored in the SpawnPotentials which would cause both to be overridden when using a spawn egg. In this case you would be as well unable to tell which of the SpawnPotentials entries get overridden in case there are mutliple entries with the same tags and values. If the SpawnData entity is not in the SpawnPotentials list then only the SpawnData entity gets modified.

SpawnData spawned	SpawnPotentials set	Changed tag
		SpawnData and SpawnPotentials
		SpawnData
		SpawnData and SpawnPotentials

Key

• SpawnData spawned = Spawner spawned the entity provided in the SpawnData tag already
• SpawnPotentials set = SpawnPotentials tag was set by a command, else the game adds this tag
• Changed tag = The tag that is changed
• = Yes
• = No
• = Does not matter

Note

For all the different situations listed above it does not matter if the SpawnData tag is set

Possible reason

SpawnData spawned	SpawnPotentials set	Possible reason
		Next SpawnPotentials entity object is set as SpawnData, so both contain the same entity object
		Changing the entity stored as SpawnData with the spawn egg
		Same entity object of SpawnData is used in SpawnPotentials

Possible solution

One way to solve this would be to have a tag called SpawnIndex (Integer) with these values:

How to reproduce

SpawnData spawned	SpawnPotentials set	Spawner command
		/setblock ~ ~1 ~ mob_spawner 0 replace {SpawnData:{id:"ArmorStand",CustomName:"SpawnData",CustomNameVisible:1b},SpawnPotentials:[{Entity:{id:"ArmorStand",CustomName:"SpawnPotentials",CustomNameVisible:1b}}]}
		/setblock ~ ~1 ~ mob_spawner 0 replace {SpawnData:{id:"ArmorStand",CustomName:"SpawnData",CustomNameVisible:1b},SpawnPotentials:[{Entity:{id:"ArmorStand",CustomName:"SpawnPotentials",CustomNameVisible:1b}}],Delay:32767s}
		/setblock ~ ~1 ~ mob_spawner 0 replace {SpawnData:{id:"ArmorStand",CustomName:"SpawnData",CustomNameVisible:1b},Delay:32767s}

1. Use the respective spawner command
2. Right click with a Sheep spawn egg
3. Inspect the NBT data of the spawner, for example with the following command (while standing on top of the spawner)

   /blockdata ~ ~-1 ~ {}