To reproduce:

1.

/give @p spawner{BlockEntityTag:{SpawnData:{id:"blaze"}}} 2

2. Place it in Creative mode.
Result: A blaze spawner.
3. Place it in Survival mode.
Result: A pig spawner.

The same also happens when explicitly setting the entity to be summoned:

/give @p spawner{BlockEntityTag:{SpawnData:{id:"blaze"},SpawnPotentials:[{Entity:{id:"blaze"},Weight:1}]}}