Seems to be know issue with ARM template conflict.
Snippet from the blog-
When using Microsoft.Sql/servers/administrators as a child resource, it’s important to have in mind that AAD admin operation is an asynchronous operation and here is where the fun begins. Although AAD admin operation returns and Http 202 accepted that doesn’t mean the operation has started, the operation was market to start and it will start, we just don’t know when.
This could result in conflicts or internal server errors with other operations we have in our template (which are synchronous).
If you have an ARM template with several resources operations (SQL database creation, setting firewall rules, connection policy…etc), and you want to use the Microsoft.Sql/servers/administrators as a child resource (and not as a parent) you need to be sure one of two things to avoid conflicts with Microsoft.Sql/servers/administrators: