When I go to the main menu, go to the config options and change some settings, it resets and won't save when I go into the game, it becomes normal. Is this an issue on my end or a bug in the code?

The in-game config does not work for some reason. Instead, try editing the config directly in the minecraft files. Look for thiccentities.cfg. Right click and open with notepad or Visual Studio or whatever text editor you want. Ctrl+F to find an entity.