It’s vsync. By the nature of how vsync works, unrelated to anything in our game, if you have it on then when fps is 60 or above, it clamps to 60. So if fps was 61 or 10,000, it would become 60. And if fps is between 30 and 59, then it will clamp to 30. So if your computer’s power happens to be such that at a given resolution, at a given graphics setting, you are dropping below 60 and you have vsync on, then yes it will clamp to 30.
So the remedy here is either turn off vsync and deal with the screen tearing that causes (on all computers, this is unrelated to our game) or keep vsync on and choose lower resolution and/or lower graphics quality such that you always stay above 60fps.