When you hold an item in your hotbar, there is a background that is displayed and in Minecraft Java edition there is no background. I know there was a similar bug report that was marked as “works as intended” (MCPE-79496) but this makes a difference between Minecraft Java edition and Minecraft bedrock edition.
Steps to reproduce:
1. Grab any item
2. Select it in your hotbar
There is an option to remove the background in Settings > Accessibility, but it currently does not work. This issue is being tracked at MCPE-79331, so I am resolving this ticket as a duplicate.