A few thoughts:
1) Clear your cache and cookies, and start all over to see if the problem persists.
2) If it does persist, this could be a bug and you may need to contact support.
3) In the meantime, it seem like there is an issue with the form and the user's cookies. Since the field is being autopopulated, the selection isn't recognizing the dependency field. Try removing the pre-populate/progressive profile option on that individual parent field (I'm blanking on if this is whole form specific, or field specific), and see if when you go back to the form it works. Again, probably not ideal to not let this field pre-populate, but this could be an interim solution while you have support look at the issue.
Hoping someone else has a better solution! 🙂