I use the latest Essentialsx and the latest spigot. and when I was afk my friend notite that my name in the tablist was strange (see picture). my config: https://pastebin.com/3m3aAiuY

Can you please give us your essentials version string. Type /essentials version and tell us the out put :)

Lol

You need to surround the value of afk-list-name with quotes:

afk-list-name: "{USERNAME}"