The issue is that anything you put in the prompt gets considered along with all the other stuff you put in the prompt. There's no way (currently) to give instructions for how to format the output that don't also affect all of the 'reasoning' along the way. So if you tell it to do everything normally, but to end every response with "Cheers", you won't just get that; you'll get different responses than you would have otherwise.

This is pretty unfortunate, because every LLM I've used has at least one tic that I find quite annoying. It would be great to be able to eliminate them. Sometimes I do, even knowing this drawback. But there is generally a cost. (Though I don't know if "lossy" is quite right, as that implies it's always a degradation. I think it's more likely to be harmful than helpful, given the models were tuned for their default state, but it is more of a random perturbation with a slight negative bias than a strict loss.)