The bug

When using the summon command, it will ignore items stacking restrictions, unlike /give and /item (MC-117005). It will cap at 64 when summoning item entities but goes beyond 64 when an entity holds an item in its hand.

How to Reproduce

1. Run the command

   /summon minecraft:item ~ ~ ~ {Item:{id:"minecraft:lava_bucket",Count:64b}}
   

   →  You get invalid stacked items

1. Run either of those commands

   /summon minecraft:armor_stand ~ ~ ~ {HandItems:[{id:diamond_sword,Count:127b}]}
   /summon minecraft:armor_stand ~ ~ ~ {HandItems:[{id:lava_bucket,Count:64b}]}
   

2. Right-click the armor stand item
   →  You get invalid stacked items above 64.