Hey vikingsteve,
If you're hosting mattermost on your local network, there could be a chance that compatibility view is still enabled for all sites in your intranet
If that's not checked, can you check the development console to see what document mode is being used by the browser? You can get to it by clicking the gear icon in the top right corner, selecting the "F12 Developer Tools" option, and then going to the "Emulation" tab of the window that opens up. It should look something like this