The Bug:

Skeletons from skeleton traps can very rarely spawn with additional armor.

This issue is incredibly difficult to reproduce as the chances of this happening are extremely uncommon, however, it's still possible.

Steps to Reproduce:

1. Summon a skeleton trap by using the command provided below.

   /summon minecraft:skeleton_horse ~ ~ ~ {SkeletonTrap:1b}

2. If no skeletons were summoned with additional armor, repeat step two again.
3. Take note as to whether or not skeletons from skeleton traps can very rarely spawn with additional armor.

Observed Behavior:

Skeletons can very rarely spawn with additional armor.

Expected Behavior:

Skeletons would not be able to spawn with additional armor.