The server is running in offline mode
What it looks like in your log
SERVER IS RUNNING IN OFFLINE/INSECURE MODE!
What causes it
online-mode=false: the server accepts any username without checking it against Mojang. Anyone can join as anyone, including your admins, and player identities are name-derived UUIDs that change if online mode is ever switched back on.
How to fix it
Set online-mode=true in server.properties unless a proxy in front handles authentication, in which case configure proper forwarding (Velocity modern forwarding or BungeeCord IP forwarding) so real identities reach the server.
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Check your own log