Could you please be so kind as to let me know what compression level you use for prince-jpeg-quality: auto, if you don't mind? Would you say the maximum compression before seeing visible defects would be around 75%? Are you using something similar to the IJG JPEG scale?
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prince-jpeg-quality
Setting a value of "auto" will not perform any recompression, so the JPEG image will be embedded in the PDF exactly as-is. Since we are using jpeglib from IJG, the quality percentage should be based on their metrics.