I received a response from Larian support 1 hour ago regarding a bug report I filed on this matter. It seems the Warlock spell slot bug has been fixed in recent patches, so now as a multiclass Sorcerer-5 and Warlock-2, I didn't have to use lvl 3 spell slots to cast the Hex. Unfortunately for Sorcerers, the bug has not been fully fixed. and the Sorcerer still cannot use Warlock spell slots to create sorcery points if he has spell slots of his own. But I believe that for the rest of the classes, the multiclass should now work correctly.
