The User - Logins Per Second alarm is raised when the average number of logins per second exceeds a threshold. This value is taken over a specific number of background collections.
This can be an indicator of poor application design. Creating a connection to Azure SQL Managed Instance is relatively expensive, and coding practices where an application repeatedly connects and disconnects from Azure SQL Managed Instance should be avoided.
While reconnecting will not necessarily slow down all users of the Azure SQL Managed instance, it will often result in poor performance for the application that is re-connecting all the time.
When the alarm is raised use Microsoft SQL Server Profiler to monitor, connect and disconnect events, and determine if a badly behaving application is causing the problem.