When you create O365 group and add team to this group you may face with the following issue: group owners and members which are added via Azure AD portal (https://portal.azure.com) are synced to team owners and members with delay. Let’s say we created group and added team to the group. During creation we specified 1 user as group owner:
If we will check now Team for this group > Manage team > Owners we will see that Owners list will be initially empty:
After some time group owner will appear in team owners:
(Note that there is still 0 owners count in parenthesis – but this is another issue. Probably bug in current version of Teams web app). Same happens also for group members and team members. This sync delay may take several hours.
In this forum thread there is suggests to use beta API for adding owners to group, i.e.:
But the point is that even with old API owners/members will be synchronized between group and team but after some time.