Hi there!
It is possible that the way your account was originally setup has a child's date of birth or that there are still restrictions you set in place before removing the account that's why! I suggest try checking your Microsoft family page at:
https://account.microsoft.com/family
Since you have removed the account from the family group. I suggest add it back as a member so that the parent account can remove the restrictions first. You may also follow the Microsoft guide here to resolve this:
In case there are no other adult accounts, you can try creating another adult and re-add your account as a child or member on the Microsoft family. Once done, using the new adult account, you can then proceed to remove the restrictions for your original account before removing them from the group.
This is because only the adult account in the family can make changes to a child's account.
I hope the information above helps. Feel free to let us know if you've tried the above.
Please note that this is a public forum, I will be responding you in short period of time and regret for a delay in response. As I am merely a fellow user trying to provide insight and information that may be helpful to others. Thanks!