Running into the same issue with the Birth Skill.The Birth skill doesn’t even show up as an option nor do the characters produce seeds. I started from the beginning and used an old save file and still no Birth Skill.
I think it's just that some of the birth skills are broken right now. Birth and Birth++ skills don't seem to exist at all, but Birth+ Does. So it seems like you just have to wait until you get a capacity upgrade and only ever use Birth+ as a workaround until it gets fixed.