Not always but often, if server is empty, for some reason the player count on server list is wrong/outdated.
For example, if I play on the server alone, exit, come view the server list I might see myself as being online on the server.
If 2 players exit at roughly the same time, sometimes they might be shown as being online, despite being already gone.
What might cause this problem? I've checked, and it happened even without plugins.
It's (obviously) online mode paper server, 1.21.10.