Here are some steps to recreate this possibly bug. And yes this is really annoying when I want to do a command block creation and it just deletes what I've been working on, if you can Mojang or who ever is reading this please fix this because it really is a inconvenience. Version of Xbox One: 2013 Xbox One. I also use a keyboard to type much faster but even without the keyboard plugged in it still does the same issue.

P.S. Sorry for the bad picture I had no other means to show you.

Step 1: open chat

Step 2: Try to type in chat with a keyboard plugged in

Step 3: Have the Xbox chat box in the Minecraft chat box