So, I eventually figured it out after confirming that not a single connection attempt had been logged by the broker.

The update in combination with the configuration of this specific installation caused another IHostedService to never finish its Initialization, leading to MyMqttHandler's StartAsync never being called and my application never even trying to contact the mqtt broker.

This also explains why there weren't any connection attempts.

Funnily enough, the setting used to configure the client-id was also involved in triggering the bug in the other IHostedService, explaining why it worked with a different client-id.

So I guess adding logging in StartAsync / what IHostedServices have been started already would have made troubleshooting a lot easier.