Hello,
May I confirm,
In your Microsoft family group, is it just you (as the Child) and your Mom (as the Organizer), with no one else listed as a member?
- Open an incognito tab and sign in with your Microsoft account here (Child account) https://account.microsoft.com/family/home
- Post a screenshot of your family safety page here.
Based on what you've described, it looks like you may not have actually left the old family group where your grandma is the organizer. A screenshot of your account's family group page would confirm who the current organizer is.