I don't know too much about UI/GUI or Bedrock Edition, but I think that you are using the "Old/Original" UI/Button layout, which might be the cause of the issue (the issue could also be that your screen size isn't commonly supported). Bedrock Edition also has a screen border setting, but your Hotbar looks normal, so I am assuming it is because you are using the old buttons. But overall, it could be a combination of, screen size, ui scale of 3, and the old ui.
Edit: Can you try using the new UI and update your post? Thanks 🙂
MCPE-160126
I don't know too much about UI/GUI or Bedrock Edition, but I think that you are using the "Old/Original" UI/Button layout, which might be the cause of the issue (the issue could also be that your screen size isn't commonly supported). Bedrock Edition also has a screen border setting, but your Hotbar looks normal, so I am assuming it is because you are using the old buttons. But overall, it could be a combination of, screen size, ui scale of 3, and the old ui.
Edit:
Can you try using the new UI and update your post? Thanks 🙂