Ah, okay, so it's not about what kind of spell slot we can use to cast spells, but to create sorcery points. I get it now!
I'm guessing that this is something that has been reported to Larian via the "Report a Bug" link up the top of the launcher? That would be the best way to get info from them about whether they see this as a bug to be fixed, or whether it's per their design.
Can't smite with warlock slots either. I have a Paladin 6/Warlock 1 and was expecting to be able to smite using my level 1 warlock slot and it is not an option. In doing some reading on reddit this appears to have to do with having normal spell slots that are a higher level than your warlock slots. This is still very much a build breaking bug.
