This bug is a hotbar bug, only for Pocket Edition, not Windows 10.

This bug really annoys me and I don't know why this has not been resolved, if this is a feature I don't like it. It's really easy to reproduce. Step 1: Make the walk and crouch/sneak button smaller (Button size slider -------). Step 2: Look at your hotbar and look, it has a slots, not 9! Thanks for reading!