NOTE: I couldn't find a ticket related to this after 30 minutes of searching and several search parameter adjustments.

Occasionally the inventory would glitch resulting in invisible items to be present or seemingly vanish completely. I have witnessed this on Xbox and Android versions in both personal and Realm worlds. This isn't easy to recreate, but does happen frequently. This mostly happens with stackable items (gravel is most famous for it) collected at any speed, but has affected Shulker Boxes and tools. Either two things can happen:

1. Item is literally gone until you leave and rejoin the world or realm but the space in your inventory is considered occupied and no other item can use it.
2. Items In a stack are visible, but you can't move the stack from the slot it's in until you either join and leave the world or realm, or you pickup another block that can join the glitched stack (in some cases, picking up more doesn't work).

You might be able to reproduce the stacked version of this glitch by using an efficiency 5 tool and haste. However, it can happen even without any buffs to speed. I will try getting a screen recording of this next time it happens.