Hey Kevin, I have seen issues with Wi-Fi on 5.8 and that improved with 5.9.0.
I have also experienced issues in a particular FW with logging. It uses BLE instensively and logging somehow was getting in the way of proper functioning. All those issues went away again with 5.9.0 and this:
SerialLogHandler logHandler(LOG_LEVEL_NONE);
Eventhough I removed all logging from BLE handlers, as I discussed here, issues remained so I turned off all logging.
I started to suspect something wrong with logging when the firmware behaved somewhat better when I had my laptop with the serial logs opened. As soon as I disconnected my laptop, things would start to misbehave - I could "see" this from the cloud. Then I reconnected my laptop, things would go back to normal. And so on. I cannot explain this phenomenon, but it may be getting in the way of your MQTT tests. Who knows.
Best,