Bundles dropping a stack of non stackable items would cause them to still be stacked when picked up

Run the command

/summon minecraft:item ~ ~ ~ {Item:{id:"minecraft:bundle",Count:1b,tag:{Items:[{id:"minecraft:diamond",Count:64b},{id:"minecraft:diamond_chestplate",Count:64b}]}}}

fill your inventory, right click the bundle, and once you pick up the diamond chestplates the item will still be stacked. This was only fixed for bundles.