vSync turns off in fullscreen mode. When switching back to windowed mode vSync turns on. Issue can be solved by enabling vSync when switching to fullscreen mode. In LWJGL3, you need to enable vSync again after switching to fullscreen mode: glfwSwapInterval(1)

How to reproduce:

1. Start Minecraft
2. Go to the video settings
3. Enable vsynс and fullscreen mode, set unlimited framerate
4. Restart Minecraft
5. Load world
6. Press F11 (switch to windowed mode)
7. Press F11 again (switch to fullscreen mode)
8. Press F3