If you hold a keybind for more than 2-3 amount of seconds, then try to switch to another keybind for a different slot, then  it will not allow you to. 

Example:

My food keybind is X

My fireworks keybind is a mouse button.

It is a muscle movement that I hold onto the X key for a few seconds while I eat, then I switch to my fireworks. It doesn't let me for a few minutes. This can cause some people to die, and even if I am eating, and I switch to my sword (1) or axe (3), it still does it.

P.S I tried looking up this bug and did not find anyone that said anything about it, but it could still be out there.

another P.S, this also happens not on fabric, i tested it and so did my friend.