The backend is not set up for the proxy
What it looks like in your log
If you wish to use IP forwarding, please enable it in your BungeeCord config as well!
What causes it
BungeeCord forwards player identities, but the backend server has bungeecord disabled in spigot.yml, or a player connected straight to the backend port.
How to fix it
Set settings.bungeecord to true in the backend spigot.yml and restart it. Then firewall the backend port so only the proxy can reach it.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log