Technically, it doesn’t mandate that. But it does mandate that no elector can vote for both a President and Vice President from their own state, so it would open up the possibility of a split administration. Which is why parties avoid it.
It was amusing to see people float Gavin Newsom as Harris’s VP pick for that reason. With that rule, and the outsized number of Electoral votes that CA has, it’d practically guarantee a Harris/Vance administration if she won.