Interesting, I've just done a test, import jpg SOOC, export as "original" and the file is indeed different, the md5sum hash is different, so I checked the exif and it's modified (which would explain that).
BUT that's literally the only change. If I take the input file and export and strip all the exif (exiftool -all= FILENAME) then the original and export ARE identical:
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So I can say for certain, Lightroom isn't modifying the pixels.
Can you share a screenshot of your export settings?
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