No bug. Just like OTB, three time repetition must be claimed. Let me find you an official link:
https://support.chess.com/customer/portal/articles/1444798
Automatic draw by repetition failed.
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Hi, I played a game where I and my opponent repeated the same moves more than thrice, yet, the game continued.
It is not the first time something like this happens. Maybe there is a bug on some piece code where it prevents reckognizing the repetition.
http://www.chess.com/livechess/game?id=939704468