The bug

When creating a bundle using commands and entering an invalid item ID as one of the items, an empty slot is displayed (in addition to the + slot that is always shown when more space is available within a bundle). This is caused by bundles treating invalid items as valid items.

When right clicking this bundle in the inventory, the empty slot is removed. When right clicking this bundle outside of the inventory, a hand animation plays, and the empty slot is removed.

A screenshot can be found below.

How to reproduce

Run the command:

/give @s minecraft:bundle{Items:[{id:"minecraft:glass",Count:1},{id:"minecraft:mojira",Count:1}]} 

Screenshot

This screenshot was taken in snapshot 20w48a. Due to some recent changes to bundles, this may not be relevant anymore. I'm planning on getting a new screenshot tomorrow.