Issue with Group email address domain when create a new Microsoft 365 Group

I have a strage "issue". When I crate a new Microsoft 365 Group on azure the default Group email address domain is @mydomain.onmicrosoft.com and I can't change it. When the group is create, the Group email address domian changes in @mydomain.com. My goal is to create a Microsoft 365 group with Group email address domain @mydomain.com and at the end of the day this is done..but I can't understand why I can't change the domain in first place and why it change automatically.

I tested the same on my environment and it was the same for me . This is because when you are creating a Azure AD tenant , you by-default get a Microsoft Domain which will be the primary domain like M365Bxxxxx.onmicrosoft.com but when you create a custom domain as mydomain.com and verify it and then make it as primary , the Microsoft domain can't be deleted and the custom domain get assigned as primary domain for the Azure AD .

So, When you create a Group it bydefault takes the Microsoft Provided Domain for your AzureAD in that dialog box and it cannot be changed.

But When you click on create , in the backend mapping it takes the primary domain like group@azure_ad_primarydomain instead of group@domain.onmicrosoft.com which is provided by Microsoft. Basically it will automatically get changed to the primary domain assigned to the Azure AD after you click on create.