Add azure security group to mail enabled security group

If I create azure security group, I can add it to mail enabled security group (created via office 365 ecp) using PowerShell.

The mail enabled security group is displayed in Azure portal. However, the "Add members" button is disabled.

The group chain (mail enabled security group -> azure security group) propagates Send On Behalf permission for azure security group

So, my question is: if SOB permission is propagated, why it is allowed to add azure group as a member only via PowerShell and not allowed in UI (including exchange admin center and exchange control panel)

Best regards,

Dmitry Alexandrov