The Bug

Before 1.11, Skeleton Traps that were activated in the Nether would spawn mostly Wither Skeletons instead of regular Skeletons riding the Horses. Since the first 1.11 snapshot (16w32a), they no longer have this behavior, still occurring in the most recent versions of the game.

Steps to Reproduce

1. Open up 1.10.2
2. Load any world
3. Go to the Nether
4. Use the command

/summon EntityHorse ~ ~ ~ {SkeletonTrap:1b}

5. A Skeleton Trap should activate, which spawns mostly Wither Skeletons riding Skeleton Horses.
6. Load up 16w32a (or any other version after 1.10.2)
7. Use the command

/summon skeleton_horse ~ ~ ~ {SkeletonTrap:1}

8. A Skeleton Trap should activate, but now spawns only regular Skeletons riding Skeleton Horses.

Observed Result

In version 16w32a and above the skeleton trap in the nether didn't spawn wither skeletons

Expected Result

The skeleton trap would still spawn in wither skeletons in the nether