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I believe that we do not have any restaurant owners on our site so I went ahead to research some thoughts on this matter.
This very thing did happen years ago as reported below:
You are correct in thinking that the restaurant will take a loss if customers just eat the fish and nothing else. What I believe may happen is that the restaurant will charge you the full price even if you do not eat the rice or anything else. Also they may just charge you the a la carte price for what is eaten.
Best though is to check with the buffet and its rules before going for a meal.