If the VRChat is launched (in VR mode) with my HMD bottom up, when the main menu (not the launch pad) is opened, it's upside down as well.
Reproduce it by simply follow the steps.
  1. Open SteamVR.
  2. Put you HMD upside down.
  3. Open VRChat in Steam library on your desktop.
  4. When entered a world, open up the menu via launch pad.
  5. Get an inversed menu.
Just did it one more time before I post this so it should still be there.