I had a similar issue on my iOS device. None of the spawners that I have encountered after the recent update spawn or show any sort of life in them (i.e. flames). I think this is a bug in my case because first spawner I encountered with this problem WAS ACTIVE before the update and went dead afterwards. Is there a way to fix??
And I already tried adjusting difficulty setting and that didn't work either.
Mine also has this issue ever since updating to the new version of MCPE.