The game currently has some keys bound to do multiple things, like Left Alt expanding tooltips, show world tooltips, or Tab for both toggle combat mode and toggle party/ character overview.

When you try to rebind those keys, the game warns you that multiple things would be affected but it won't let you bind the new key you are choosing to those multiple options as well. So if you for example just would like to swap those two keys around, so Tab shows and expands tooltips and Left Alt toggling combat mode and party / character overview, the game simply won't let you do that since the new key is already bound to one of those two things and can't have multiple bindings like it had before.