Hi,
That is by design the admin will need to be part of the role role_developer Please add your admin_user under membership as shown below and give it a try.
So basically we can explicitly assign rights or ownership of DB object to the required user i.e admin user
for ref --> please check https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/how-to-create-users
Please let me know if this helps or if you need any additional information.
Regards