Since I updated to version 1.18.30.04 some slot boxes do not show the item in the player's hand when selecting this box. 

I attach the screenshot, I had a netherite pants in the third box, when selecting this box it did not let me put on the pants and it did not appear in my character's hand either. This error has happened to me more than 5 times with different items, and for now I have not found a solution other than leaving the world and re-entering so that the slot boxes work well.