Don’t waste too much time on a wild goose chase. The “issue” isn’t really one since either vorpX (or the Oculus runtime whenever it decides to throttle on its own) interpolates frames, so in the headset you actually always get 90fps.
Monster Hunter World is a tough cookie performance wise. Better spend your time finding game settings that work acceptable for you.
BTW: This was one of the games where I got pretty huge benefits out the upcoming fast sync option in my testing. Your mileage may vary, so I won’t promise anything for your particular setup, but if you are lucky you will experience the same when you enable it in the next vorpX version.