Unknown cause
The 'prev_participants' value is set around line 108:
'prev_participants': game[id]!==undefined?game[id].participants:null
Example (channel and user IDs redacted):
{ '(redacted)': { inProgress: 1, participants: [], correct_users: [], correct_names: [], correct_times: [], prev_participants: null }, '(redacted)': { inProgress: 1, participants: [ '(redacted)' ], correct_users: [], correct_names: [], correct_times: [], prev_participants: null } }
EDIT: may be related to game messages not sending when users start a game
EDIT2: Confirmed still present in 6c3fae5