1. Join the instance with many people at the same time (using a portal may be more reproductable).
  2. Sometimes joined users are unintendedly blocked locally and could not see and hear them. (can be one-sided blocked: Player A can see Player B but Player B cannot see Player A)
  3. See player list at "Here" tab in Launch Pad. If the unintended blocking is happened, blocked list will be shown with bugged player.
  4. Rejoining the instance either blocking and blocked player resolves the problem and can see each other again.
This reproduction is not guaranteed but often happens stealthy if moving through a portal, confusing our how much people are came and don't realize the unintended blocking is happened.