The bug

Players are able to pick up items from the floor whilst on a horse but Skeletons are unable to do the same and need the help of a piston or dispenser.

How to reproduce

1. Run:

   /summon skeleton_horse ~ ~ ~ {Tame:1b,Passengers:[{id:"minecraft:skeleton",CanPickUpLoot:1b}]}
   

2. Drop a diamond sword under the horse and note that the skeleton doesn't pick this up