Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello
When you went from Family to Premium, you go from fewer features to more features. According to this article: https://support.microsoft.com/en-us/office/switch-between-microsoft-365-subscriptions-3fcc1efc-2722-427f-8efa-db94b9b0a36b
So basically your old family plan was converted to Premium, and the remaining days on that family plan was automatically converted to the Premium plan, which you later canceled and refunded. So, it is not like you still have the family plan out there, I hope that makes sense.
If the refunded amount doesn't appear to be reasonable or you have more billing issue, get in touch with billing support using this specific method:
Go to https://support.microsoft.com/contactus, Type "Billing" and press Enter. On the next screen, click "Contact support" at the bottom of the page, then choose "Microsoft 365" > "Manage my subscription". You’ll then see the option to chat with a Microsoft support agent who has access to your account to help review the issue.