1.) ItemJoin-v5.0.5-RELEASE

2.) CraftBukkit version get-Spigot-800b93f-8160e29; API version 1.15.2-RO.1-SNAPSHOT (But I also used the server with PaperMC, there's the same problem)

3.) I used this plugin to lock down the equipment slots. I kind of put in all the necessary flags for the items so that the players can't do anything. But I found a way to move them. If you use the numbers on the keyboard, the item will move to the hotbar, but it doesn't work the other way around. To return the object to its place, you have to die. Is it a bug, or did I not write some flag to the items? Video: https://youtu.be/W9vdC9dhSd8
(There may be errors in the text. I used a translator)

4.) https://pastebin.com/RzvC3i8v (I didn't change the rest of the files)
5.) https://pastebin.com/zmiM3TjH

Ah, I've already figured out that there's a theme to this problem, and it's solved in snapshot

Yes, sorry for the runaround, the release of v5.0.6 will be coming out soon. #259