Yeah, I can confirm this one. I built a blaze farm in the nether about a week or so back. When I went back to it today it worked like a charm. . . For about 15 minutes. Now the spawner just sits there. It is still showing as active with the tiny blaze spinning around and around but nothing spawns from it. Other blazes still spawn sporadically around my nether fortress just not in any of the spawners. I tried placing down light sources then removing them, but nothing seems to get them spawning again.
So much for my glowstone project for now.
I recently reset my nether and now it doesn't even spawn with blaze spawners or chests
The only way my world can get horse armour is via nether fortresses.