This issue takes place on both the 1.21.1 and 1.21.4 versions and doesn't matter if you are running NeoForge or Fabric/Quilt.

Steps to reproduce:

• Install latest version of ImmersiveUI
• Set "selectedScreenType" to "EXPERIMENTAL_V1" in the accessories mod config file, or select the experimental screen in-game.
• Open the accessories menu.
• Hover an item over any accessory slots.
  -> Notice that the texture for the hovered equipment slot renders over the held item.

This issue also effects the button to open the accessories screen, as seen here:

Interesting though may take some time to figure out exactly why this is occurring besides other mod messing with Z Index ordering which is going to be hard to fix without knowing what is happening. The button is known issue with its z index I believe without the mod so something I need to get around to fixing when I get time.

for me the highlighted square is even in front

the hover settings in the config don't apply to the experimental screen aswell, I tried fixing it like that but it didn't work
nvm my mistake, I thought it meant the slots itself