Huh? The Leo scene isn't bugged, it's just kinda hard to understand how to obtain
You need to clear all scenes with Flare first (the last character in the yariman dex), after that, you can remove the cheated yarimon from your team in front of the Central Tower by interacting with the blue and yellow flame
Once you've done that, you can do the Leo event
I do agree tho, I would've preferred an actual "main quest" for the DLC, but to be fair, there isn't much to explain in the story, most if not all questions are already answered