The problem is simple. The game only distinguishes between spell slot levels but not how the slots are acquired. This means that Warlock slots are treated as regular spell slots in the game. And there's simply no option to choose which level 1 slot to use!
It was simply forgotten that Warlock slots have different rules. In D&D, I would even allow this only because coffelocks are absolutely OP there, but in Baldur's Gate 3, due to the limitation of short rests, this isn't the issue. (Unless you go overboard and give each of your companions 2 levels in Bard.)
