よくある問題Equipped heirlooms do not provide XP bonus
The experience bonus has been replaced with a new set bonus feature to help your character throughout the leveling experience.
The new set bonus can be unlocked from all Heirloom pieces except weapons and trinkets. To activate the set bonus, the equipped item must be higher level than the character equipping it. For example, equipping a level 34 item on a level 39 character will not unlock a set bonus.
More information about this patch change can be found here.