Richard HM

I can confirm this issue affects 0.12.1.
Running Android Kitkat
Difficulty is set to max level on survival. I have not tried to change to creative to see what happens yet.

I have seen 2 types of dungeon spawner issues.

1) there is just an empty spawner cage. No fire and no mob. I have seen this once.
2) there is a spawner with fire particles and no mobs inside cage. No mobs spawn inside the dungeon. Seen this on every other dungeon I have come across.

I have also seen an empty spawner in the nether, just with fire particles but no mobs inside. On the same map.

Seed for this map: 1307477

Head straight ahead from spawn until you come to the village. Go through the village and head right slightly (east) cross the river and there are some visible caves. It is easy to navigate down to the dungeons. They are numerous.

UPDATE:

Created a new world in 0.12.1 and this problem doesn't exist. Could this be an upgrade issue with worlds created prior to 0.12.1?

I concur with this issue. Seen in 0.12 update from play store running Android KitKat.
I created 5 portals 18x16 blocks to lure pigmen to the overworld. None appeared in the overworld, but my nether world portal (regular 5x7 portal) is now swamped by pigmen!