If you create a Bundle with more than 64 items inside (Like with the /data command) and attempt to add anything additional to it, it will add a stack of air for every time you attempt.
Rightclicking with an empty mouse will remove these air stacks one by one as well.

Repoduce

• Run the command

  /give @s minecraft:bundle{Items:[{id:"minecraft:egg",Count:64},{Items:"minecraft:egg",Count:64}]}
  

  /give @p bundle[minecraft:bundle_contents=[{count:64,id:"minecraft:spruce_log"},{count:64,id:"minecraft:stone"}]]
  

• Try to place another item in the bundle