LLM-generated summaries of customer feedback increasingly shape purchasing decisions and organisational strategy. However, we find that such summaries often do not stay faithful to the customer voice, potentially undermining the validity of any downstream decisions made from them. In particular, through a systematic evaluation of 50,000 synthetic customer reviews across 50 domains and 11 models, we demonstrate that the same topics are amplified across different models, regardless of their prominence in the actual reviews. More strikingly, models continue to amplify these same topics even when they are entirely absent from the input, and the topics that are amplified correlate with those the model independently rates as most important. Our findings were validated on real-world reviews from four domains. Furthermore, no intervention we tested, including scaling to frontier models, prompt engineering, and varying temperature, summary length, and input size, reduces the bias to acceptable levels.