I was on my survival world on peaceful and I went to a fortress and found a blaze spawner spot and there was nothing there just air where the spawner should be. so I turned it on easy and it started spawning blazes like normal but there was still nothing there.

Seed: chancecash

location: 336 78 -112 location of the fortress but not the spawners just look around it affected all of them.

To recreate this it happens so much to me just go on switch go to the nether find some fortresses. They sometimes dissappear after playing and logging of for a while so just quit then rejoin a few times and then go in a nether portal out of the vicinity of it, then do in the vicinity of it.

Update this happens to all spawners someone in the comments said stronghold and I have seen some dungeon spawners missing