VSC the copy line Down doesn't work and when i trie to edit the key combination it not resolving

I try do copy down line with the shortcut : Ctrl + Shift + alt + DownArrow but it not working. I have a small keybord (65% => so i have to press the FN key to have the Down Arrow) and when i trie the combination keybinding by editing in VSC, the key down Arrow is not taken into account.it like i press only "Ctrl + Shift + Alt".The key arrow Down work well if i press only the combination FN + K.observation: the FN +K give "Down" in the keybinding editor of VSC and not "DownArrow" by default in VSC.