The GSit plugin is installed on the server and the option PlayerSit: allow-sit: true is enabled.
This mode allows players to board other players. If a player starts flying with a passenger, it allows players to cross the border of the world without restrictions.

Can you share more details such as your config file for both ChunkyBorder and Gsit as well as instructions for how to "board other players". Not familiar with this.

ChunkyBorder config.txt
GSit config.txt

The allow-sit: true option in the GSit plugin allows you to sit on another player. You need to right-click on another player and you will ride on top of him. A player can fly on elytra or just use /fly and fly off-world. The plugin's restrictions no longer apply to it.
Most likely, the GSit plugin uses some special mechanics to move players in this mode, perhaps by teleporting them so that both players move synchronously, and therefore the plugin stops responding to the passage of the world boundary.