When I try to change selected item whilst sneaking without going down (for example I am placing a sign on a command block while I am flying) to 6, i can't do that.
Every slot works, scroll for changing selected slot too, but the number "6" on my keyboard does not.
Cannot reproduce. This is most likely caused by key ghosting. See https://www.maketecheasier.com/what-is-keyboard-ghosting/