Is it normal? Yes, depending on the balance of power in your store, and it sounds like your overnight boss has more pull than your HR, which is not good for you. If overnight at your store is difficult to staff and dayside isn’t, they can deny your availability change. If they haven’t said “needs of the business” as justification, be prepared to hear it, since that is Spot’s go to justification for denying/requiring anything.
There’s not a lot you can do about it through Spot. You could appeal to your SD, but its a gamble which may not turn out well, and would really put you on the overnight boss’s bad list. Do you think your overnight boss denied your availability change because she wants/needs you back, or because she wants you to quit? If it is the former, you may have to find an overnight babysitter to keep your job. If it is the latter, you might want to extend your loa, if possible, and start looking for another job. Good luck, hope things turn out well for you.