A minimal, reproducible example of the issue with mocking IQueryable.ToListAsync() from the MockQueryable package. The issue in the MockQueryable repository explains the error.
In this solution, the error is as follows
System.InvalidOperationException : The source IQueryable doesn't implement IAsyncEnumerable<MQ38.Accessor.EntityFramework.Models.Event>. Only sources that implement IAsyncEnumerable can be used for Entity Framework asynchronous operations.
The exception is thrown on the queryable.ToListAsync() call in the EventManager class.
The EventManagerTests class sets up the mock IQueryable. This class contains the failing test.
The EventAccessorTests class sets up a mock DbSet. This class is able to pass its test.