I believe it's a tiny bit bugged at the moment. You're going to have to play around with it. Like trying turning your server offline, setting the difficulty to Easy via settings, clicking Save, starting your server, turn it offline again, then repeat the process but set it to hard until it works.
You can also just try doing /difficulty hard, doing /save-all, and restarting your server. Best of luck to it working, though! If it doesn't, you can always install the Multiverse plugin and change the difficulty using commands from the plugin and that likely should work. The command is /mv modify set difficulty hard <world>