Can you say a bit more about the problem you're facing?
In my game, I have Wyll mutliclassed as a warlock/oath of vengeance paladin. At the moment he only has level 1 paladin spell slots and level 2 warlock slots, and when he gets the opportunity to smite then I get the choice of whether to use his level 1 paladin slot or level 2 warlock slot (I have smite set to an "Ask" trigger). And when I select a level 1 spell (whether it's a warlock or paladin one), I get the opportunity to choose whether to cast it using my paladin level 1 slots or warlock level 2 ones. So, so far so good, but I assume your problem is different. Does it occur when you have warlock and other types of spell slots at the same level, which I've not yet experienced?
