This bug is related to the Xbox on-screen keyboard changes when typing in the chat box.
It is related to this bug: MCPE-159777

However the post created for that issue does not highlight how destructive this bug is to anyone who is playing Minecraft on an Xbox using a Keyboard. It is quite literally impossible to type out a command with any sort of reasonable/acceptable speed and efficiency, to the point where it's just not worth the fuss.

This bug has been around for over a YEAR now and it is unacceptable, I haven't been able to play Minecraft in the way I like to for the longest time, and my passion project is in purgatory. I seriously need the bug fixing staff to understand how frustrated the Xbox map making and command community is with this issue, please, please, please fix this, it should be way more of a priority than whatever it is now.

How to recreate the issue.

1. Enter a world with cheats/commands enabled.
2. Use an xbox with a keyboard plugged in and type "/give a"
3. Press the TAB Key to cycle through the autofill suggestions shown on the left side of the screen.
4. Result. The game does not autofill anything, and instead thinks you're using the on-screen keyboard and not the chat box in-game.