The Bug:

Saddles don't apply attribute modifiers when worn by entities.

Steps to Reproduce:

1. Summon a horse equipped with a saddle that increases movement speed by using the command provided below.

   /summon minecraft:horse ~ ~ ~ {Tame:1b,SaddleItem:{id:saddle,Count:1b,tag:{AttributeModifiers:[{AttributeName:generic.movement_speed,Amount:1,UUID:[I;1,2,3,4]}]}}}

2. Mount the horse, move around with it and observe its movement speed.
3. Take note as to whether or not saddles apply attribute modifiers when worn by entities.

Observed Behavior:

Saddles don't apply attribute modifiers when worn by entities.

Expected Behavior:

Saddles would apply attribute modifiers when worn by entities.