This happens rarely on singleplayer, but occurs much more often on servers. When a piece of armour is equipped via the hotbar (on touch controls, holding down the screen) and a different hotbar slot is selected immediately after, this lag involuntarily switches the selected slot back to the original slot. As shown in the clip, when I put on some armor and tapped on another slot immediately, I was switched back. This is extremely annoying for fast-paced minigames such as skywars. I hope my description is clear. Thank you.