How to Recreate:

• Use the command to give a chest with an extended bundle and a pickaxe.
  (text file with command linked in attachments)
• Drag the pickaxe onto the bundle and right click the bundle, with the pickaxe on the cursor.
  + You'd get 5 pickaxes, and the bundle gets added with an empty slot.

Happens in Creative,Survival and Adventure modes.

The Bug:
This bug seems to only work on custom bundles (ones made with commands to exceed the item limit), but please fix this!

Why should Mojang fix this:
 - With working custom bundles, mapmakers and datapack creators can use it to make so many awesome features with them.

Extendable Bundles being one of my ideas, and it's affected by this bug hard. And I can't fix this myself.
You can't fix something that's hardcoded. Unless you're the dev.