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When you cycle between two items, the hotbar will highlight the item you switch to, but the in-game hand will hold on to the original item. For example, if you're holding a sword in slot 1 and switch to the flint & steel in slot 2, the hotbar will highlight the flint & steel, but you'll still be able to deal damage with the sword and the sword will still be displayed in your hand. This basically means the switch between slots never occurs, so it's kinda difficult to cycle between slots quickly.

I have only been able to get it to happen when one item is used to fight/mine while another item is just 'used' (like a block, fishing rod, or lava bucket, and not an iron ingot or inc sac), although I haven't really done much testing at all so I don't really know for sure.

Could be somewhat related to another bug I found: https://bugs.mojang.com/browse/MCPE-29797

And this has actually been around for a very long time, so I apologize if I'm just reporting a game mechanic.

How to reproduce :
1. Hold an item that can be used in Slot B and hold an item used to fight/mine in Slot A
2. Use the item in Slot B, quickly fight/mine with the item in Slot A, and repeat

If that was too confusing, just do what I did in the video. It shouldn't take more than three minutes to reproduce the glitch. (I got it to happen on my first attempt).

Here's the video showing the glitch (version 1.2.8)

https://www.youtube.com/watch?v=4453reTbc-s

Same footage, less cringe:

https://www.youtube.com/watch?v=h5UXAazaBRM