After execution of an async receive handler in an actor suspends the mailbox so that the ReceiveTimeout messages are not delivered, until the next message resumes the mailbox. In a following example:

public class MyActor : ReceiveActor
{
    public MyActor()
    {
        Receive<string>(async _ =>
        {
            var result = await DoSomethingAsync();
            Context.SetReceiveTimeout(TimeSpan.FromSeconds(1));
        });

        Receive<ReceiveTimeout>(_ =>
        {
            Console.Write("Timed out.");
        });
    }
}

after sending a string message to an actor initially, even though a receive timeout is configured, the ReceiveTimeout handler is never called.

Using a synchronous handler (replacing await DoSomethingAsync() with DoSomethingAsync().Result) or sending any message to Self in the async handler both seem to fix the problem.