I went to a blaze spawner in my single player world, waited a few minutes and realized that no blazes have spawned, I decided to start a new world using the same seed to see if there was something wrong with the world I was playing in and the spawner still didn't spawn anything in the new generated world. I took the same seed into 1.9 pre-release 1 and the mobs spawned so it is a new bug that happened in pre-release 2.

I also checked on a skeleton spawner in both versions, in pre-release 2 it's broken and works fine in pre-release 1.

I assume zombie spawners and spider spawners are broken as well.