If you fix up the springs in the mansion, it opens the option for the birth, Birth + and Birth ++ Options
The game is very glitched at the moment, as there is numerous things that need to be fixed. The amount of it has pushed back the date of the next patch, along with other things going on with the content creator that is not my place to say. But, the patch is coming soon; I know that with absolute certainty.
It would be a mac in my case but unlikely, as when I emulated the previous version on it the hot-spring trick worked for Leah too. Might try a fresh download to see if it's just on my end, but I'm not getting actual errors so the game is probably just not properly setting the correct flag when changing costumes.
Wait. You're talking about the spring in the fairy forest right? Of that's the case, then people have reported that one. My issue was that there are specific dialog from the last update that never showed up in this update, as I thought it was glitch. The previous dialog from the last game made it immersive in my personal opinion, or the choice of having different dialog coming from different characters; instead of ignoring the obvious. I found that very interesting, which is why I like the series.