Run this code on any given block:

 block.setType(Material.MOB_SPAWNER);
 CreatureSpawner spawner = (CreatureSpawner)block.getState();
 spawner.setSpawnedType(EntityType.ZOMBIE);
 spawner.update();

• Watch it turn into the requested spawner type (zombie).
• (Optional) Watch it remain unchanged for any timespan as long as it doesn't spawn anything, even across server restarts.
• Watch it spawn the requested mob once (zombie).
• Watch it turn into a pig spawner immediately.

This used to work just fine, so it is reasonable to assume that one of the recent patches to BlockState broke the CreatureSpawner, or I'm missing something.