Mob Spawners No Longer Spawn

I had many mob spawners in the game - which I use for farming. Since updating I've discovered three new spawners that were empty (with no flame). I thought it might be deliberate at first but after the third I thought it might be broken. I went back to previous spawners and they had stopped working - they still had the flame but no mob inside and certainly not spawning. The only thing I can think of (other than upgrading my phone from an S3 to S5, and the newer MPCE being installed) is that I turned it down to peaceful for a bit recently and then back up difficulty. Does this break all spawners or is it the new 12.1 update?