In my last game, it suddenly appears as a draw. My opponent or me did not offer and we did not make any repetition. Why is that?
looks like a bug
Maybe a bug. I see no other reason. Not a 3x repetition, or no draw offer. I think it is a bug.
You did not leave the game (even for short time) or lost connection? If the server finds a game where no player is present, it may declare it a draw.