I created a Team site with an O365 group and it automatically creates the 2 groups, Owners & Members. If you elevate someone from a Member to an Owner, it is reflected in the Group membership (Azure, Teams and Outlook). However, this 'Owner' group is not showing in the SharePoint Owner group under Advanced permissions. How does SharePoint know the user has Full control if they don't exist in the SharePoint default Owners group? There seems like a little disconnect here.
The 'Owner' group is in the SharePoint Owner group. However it is hidden by default.
To show the group:
2.Scroll down to the view, edit the default "Detail View"