Azure DevOps - Can't remove AAD User from Team

in Azure DevOps, we're assiging all our team members to Azure Devops using aad groups (as shown in screenshot). In order to fulfil that, each Azure-DevOps-Team contains a aad-group:

However, i can't delete all "aad user" from the team "Project admin". As shown in the screenshot, the user "BAR" can't be removed and the "Remove"-Button is disabled.

Any idea how to remove this user?

According to the official doc,

Each team must have at least one administrator. To remove an administrator, you must first add at least a second administrator.

In your screenshot, we can see the user BAR is the only admin in this group. To delete the user, you can add the AAD group FOO as the team admin. Then you can delete the user in the team.

Steps:

In the group, the only admin can't be deleted.

Add the group as admin in settings:

Now the user can be deleted.

At last, the group can't be deleted.