Whenever playing singleplayer mode, I keep being booted out of the world with the message "Server Closed"

NeoForge, 1.21.4, 2025.2

Screenclip of it: 
https://drive.google.com/file/d/1w8d-AfX3c8Y6S4ONkIryj4iGzqv3dafz/view?…

Anytime when loading the instance, I am able to only fly around in a world for so long before the game says that the "Server Closed, Back To Server List" and I am booted out from the world. This is on singleplayer, not the test server. I'm also unable to close the game itself after this happens; it only closes if I either close Mcreator altogether or click on the cancel gradle task button.

I'm working on a video right now and this issue is making it nigh impossible to progress after already putting tons of work into it.