So is it the sorceror spell-slots that are the same level as your current warlock ones that you're not able to select specifically the warlock spells for? As I mentioned, I'm currently able to pick for my multi-class Wyll, but he only has paladin level 1 slots as yet.

Eg, in the below screenshot, I'm able to choose to cast Bane (which he has as an oath of vengeance paladin) using either Wyll's warlock level 2 slots or his paladin level 1 ones. So, as you say, I can choose to use these rather than his level 1 paladin slots, as his warlock slots refill on short rest.

[Linked Image from i.imgur.com]

Am I misunderstanding your issue? Or are you seeing something different for a multiclass warlock/sorceror?

"You may call it 'nonsense' if you like, but I've heard nonsense, compared with which that would be as sensible as a dictionary!"