The DnD5e rules regarding warlock multiclasses is that you can use warlock spell slots and normal spell slots interchangeably, but you need to keep track of warlock slots separately because they recover on short rest. From my understanding, in BG3, you can use warlock slots for non-warlock things, but normal slots of the same level are prioritized first unless using a warlock spell. It should be the opposite. Because warlock slots regen on short rest, there is no scenario where you would want to use a normal slot first instead of a warlock slot.
Please either make it so that using slots at the tier of the warlock slots always uses them first, or put the warlock slots as an extra option so you can always choose to use the warlock slots.