I have no idea how to reproduce the bug but I'll list what I did before the bug occured.

1. I was creating some buffet worlds, just to explore them. I had a texturepack enabled.
2. I created a buffet world with the nether as biome.
3. Suddenly I noticed that all zombie pigmen were completely blank.
4. I disabled my resourcepack, but the pigmen were still blank.
5. In other worlds, they appeared blank too, and in one world they had the texture of a cyan shulker.
6. I quit and rejoined the world, and the pigmen appeared blank again. So did all cyan shulkers.
7. I quit my game and restarted it again and since then I've been unable to recreate the bug again.