If you have a Mouse Key and rebind a hotbar slot with it is it impossile to put an item fast sorted in that hotbar slot by using the mouse hotkey or the original horkey for the hotbar while you are in the inventory.
I probably wrote many things wrong so, here is a version in german if that can somehow help you to read it.
Wenn mann einen Maus hotkey auf einen Hotbar slot einstellt kann man im inventar einen Gegenstand nicht mit einem Hotkey auf diesen slot den man eingestellt hat per hotkey (sei es der maus hotkey oder die jeweilige zahl) bringen
That is intended, you instead use the key allocated in the Settings to move an item into that slot.