Server does not properly disconnect Players

When a Player disconnects from a self-hosted server they remain online on the server for an additional 20 to 30 seconds before a timeout occurs instead of a proper disconnect.

In the Server-Console it says "Player lost Connection: timed out" instead of "Player lost Connection: Disconnected".

Players that want to reconnect before they time out result in an immediate timeout serverside.
After the timeout has occured Players can reconnect normally.

This Problem occurs on both modded and vanilla Servers.
This Problem occurs on any 1.16 Version of the Game.
The Problem is not easily reproducable as I have not yet encountered another Server with this Problem.