Hi! My name is Ben a fellow member here in the community and hope you are doing well!
It seems you reached the support already, they should be able to check it but let me try to offer further help. When you sign in your account via https://account.microsoft.com/family/home do you still see the parent account on the list? Or if your account is still showing as part of family group?
Before I can see the email address on the same page but here is a workaround I found out to try checking what email address the family member you have is using.
First, go to this page and sign in your account https://outlook.live.com on the left pane you should see a folder or name titled "Your Family" after clicking it, click "Your Family" again which shows members > Select the name of the member you wanted to view > Then it should display there the email address of the member you clicked.
Let me know if this works for you and we will try with the best of our knowledge to offer further assistance.
Kind regards,
Ben