When there are unstackable items inside an overfilled bundle, the bundle will behave very weirdly. When I take out 1 unstackable item, everything is fine. But when I rightclick the bundle again, with the unstackable item still in my cursor, it adds 8 items to the stack. It also inserts an empty slot inside the bundle.

1. Give yourself an overfilled bundle

/give @p bundle{Items:[{id:"iron_helmet",Count:1b},{id:"iron_chestplate",Count:1b},{id:"iron_leggings",Count:1b},{id:"iron_boots",Count:1b},{id:"iron_sword",Count:1b},{id:"iron_pickaxe",Count:1b},{id:"iron_shovel",Count:1b},{id:"iron_axe",Count:1b},{id:"iron_hoe",Count:1b},{id:"cooked_beef",Count:64b},{id:"iron_ingot",Count:16b}]}

2. Take an unstackable item out
3. Rightclick the bundle again with the item still in the cursor
4. Bundle will begin behaving weirdly