Why is parchment paper burning under pastries?

The baker is experiencing brown burnt spots appearing underneath pastries baked on siliconized parchment paper. The spots are of varying sizes and appear on the pastries themselves. The issue only occurs with pastries and scones and not with other baked goods. The trays and parchment paper used are the same as usual, and the oven settings have not changed. The pastries and scones are frozen and defrosted overnight on the same paper and trays. The baker is concerned about whether the spots are harmless burnt spots or indicate chemical contamination. Photos are available but cannot be uploaded.