When both warlock and other spell slots are avaiable (in my case multiclassing paladin to warlock), the other spell slots are used to cast a spell (such as smite), even if the action is triggered by clicking on the warlock spell slots and selected from there. The spell can (only) be cast using warlock spell slots, once all other spell slots have been used.
I don't know exactly what causes the bug but it's easily reproducable so I'm just thowing it out here to make sure its visible and noted earlier rather than later.
Having a blast with the game, keep up the good work ^.^
Cheers
