This is not a new issue, but now I think I have understood fully the reason.

1. With RTX ON, the game is likely limited in performance and could go below 60 fps
2. With VSync ON, as the framerate drops, it triggers something and further limit the frame rate to 30 fps
3. This particularly easy to happen as the light level is slowly changing during evening/dawn. There is a point where suddenly it becomes bright or darker, and that is the point framerate drops from 60 to 30 fps.
4. To recover temporarily, toggle fullscreen in and out by clicking F11 twice. This solves it until it happens again, likely triggered by 3

This should probably recover after the change, but this was not the case due to some kinds of bug. Disabling VSync can solve this problem completely.

Could you 

1. Check if the above is reproduced
2. Provide a simple toggling switch in game to disable VSync, instead of relying on modifiying the option.txt?