The bug

Upon giving yourself a bundle with an invalid item ID used as one if its contents (through commands), the slot that supposedly contains the invalid item is displayed slightly brighter than the rest of the slots.

This is perhaps best demonstrated in a video: https://youtu.be/1djCe6tQrOU

Discovered while testing MC-207048.

How to reproduce

1. Run the command:

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

1. Hover over the bundle within your inventory, and compare it to a standard empty bundle
   Notice how the bundle you've obtained contains one slot that is brighter than the rest