Hi – Do you have both the Rift & Vive installed in one machine (software at least)? Thinking some HMD availability check/glitch could be causing that random behaviour. Also, if you’re using another VR Pawn, perhaps from the template at some point and it’s set as the default in the project settings?
Things you can check is that the Pawn has “Always Spawn, Ignore Collisions” setting on it, and “Auto possess, Player 0” if this is a single player game/experience and make sure that you have “Enabe HMD” node somewhere in Begin Play (Level BP, Controller actor or Pawn BP) to ensure your HMD is activated and is critical for Packaged projects. Hope that helps.
Cheers.