I was testing things on a new Flatworld with experiments on and found that the /hud command has some errors. I used an alt account on my phone to join the same world and used "/hud @s hide all" and it hide all hud items except the attack and place/use buttons. I was using the "Joystick & aim crosshair" controller mode, so it probably didn't realize there were more buttons to hide.

I also noticed that even though I ran the command for only my Iphone account, it hid all hud items on my main account as well.
I then created a new world without the experiments and it still had the hud items hidden, even though there was no /hud command on that world. I had to go back to the main world to reset the hud.("/hud @a reset")

Steps to reproduce:
1: Using your account, create a new world with all experiments enabled, make sure other players can join.
2: Use an alternate account or a friend’s account to log into Minecraft on iOS.
3: Join the world hosted by your main account.
4: Change the controller mode on your iPhone to “Joystick & aim crosshair”.
5: Run the command “/hud @s hide all”.

Expected result:
Every hud item gets hidden.

Observed result:
Every hud item gets hidden except the attack/destroy and place/use buttons.