That may be due to me writing "Windows 10 edition" when I meant to write "Pocket Edition on Windows Phone 8.1". Sorry... Can you perhaps try in that version? I just stood in front of the spawner for some time, no drops at all - with the exception of one zombie who wore armor, which dropped beautifully.
Windows 10 edition
As far as I see, the mob spawners work again in 0.12.2, but the spawned monsters (zombies in this case) don't drop any loot, not even rotted flesh.
Same problem here after updating to latest windows phone version (v0.12.1).
Device: Lumia 925 with Windows Phone 8.1
I tested a little further, and after opening the spawner chamber, I found out that the problem is not the spawner, but the mode of death. When attacking the zombies, they drop loot, when they die in the chamber's water trap, they don't. I have not tested with other mobs yet, but maybe monsters no longer drop anything when drowning? Or is that intended? It makes sense that drowning monsters do not drop XP.