OK, did you give Mula the 30 flowers during the day? Did she not show up? Did you just say, “Here are the 30 flowers!”? That was a display glitch that’s been fixed. But once you’ve done that, have you completed the quest where you introduce Coni to Mula? Once you’ve introduced Coni to Mula... That will trigger the final event. I know it might seem a little confusing, but normally you talk to her, and once you’ve chosen the right dialogue in the bath... “You can't do everything on your own” activates the quest, so if you go back to see Coni, there's a new option: “I have something to ask you.” Once you've done that and introduced Coni to Mula, you can go back and take a bath with Mula, and there will be a new scene. Sorry, this scene is a bit tricky.
If you're not sure, check the steps in the walkthrough on Patreon.