ManagedMqttClient Issue with sending message just after Start.

Hello,

### Describe the bug
I have the following case:
One server, one client (producer), another client (consumer).
I start the server.
I start the producer and I send immediately a message with retain flag.
I start the consumer and subscribe to the topic.

I am facing a strange behavior where sometimes, the retained message is not received by the consumer.

I am able to work around if I add a Task.Delay(1) between the start of producer and the enqueue of the message.

### Which component is your bug related to?

- ManagedClient v4.3.1.873

### To Reproduce
Open the attached visual studio solution, execute the unit tests.
There is 2 tests, one where the server is restarted at each loop, another where the server is started only once.
I have Theory, (one with Task.Delay(0) failing, one with Task.Delay(1) success)

### Expected behavior
A reproducible behavior without Task.Delay or a way to be sure that initialization is ok.

### Code example
See attached solution.

[MqttNetTester.zip](https://github.com/dotnet/MQTTnet/files/12729906/MqttNetTester.zip)