I've been super busy with no time to look further into this yet. The Minecraft plugin is open source. Would you be able to look at it and determine if it should be sending properly formatted messages for Mattermost? https://github.com/CircuitSoftGroup/SlackMC
Of course it could be a firewall issue or something else entirely too, but knowing if it's at least setup properly on the plugin side would be a good starting point.