UPDATE 2016-09-20: Adobe released Lightroom CC 2015.7 which includes a fix for this issue. I have confirmed the bug no longer exists.
For the longest time I always wondered why my photos with pure white (255,255,255) backgrounds were not showing up pure white on my blog. Sometime last year I stumbled across an old Adobe support forum post where someone complained about how photos exported from Lightroom to JPEG using the built-in LR watermarking feature were coming out 1 less integer value on all three channels (red, green, and blue).
I can’t find the forum post anymore, but the bug is still present and it still pains me that this is still an issue with the latest Lightroom version (I have Creative Cloud).
Addendum: I located the post in during a deep search of the Adobe Forums. The original post goes back to 02 August 2012.
Anyway, I made a 600×600 pixel pure white image (PSD) and the color picker in Lightroom shows 100% (255) across all three channels.
You can’t really tell there’s an embedded image above because there’s no border and my blog uses white backgrounds.
But if I take that same pure white PSD, import it into Lightroom (reference library) and then export it with a basic watermark (using the Lightroom watermark on export feature), the resulting JPEG is not pure white. The image is now RGB 254,254,254 (or with the LR color picker, 99.6% on all three channels).
You can now probably make out the above 600×600 screenshot because the 254,254,254 is off-white compared the the pure white background of my blog webpages.
To take this even further, if I import the exported JPG into Lightroom and export that to a JPG using the same watermarking feature, the resulting image is again 1 integer value less (darker) across all three channels (253,253,253 or 99.2% across).
Now you should definitely be able to make out the above 600×600 screenshot because the 253,253,253 is visible over the pure white background of my blog’s webpages.
I have no idea why this bug still persists, but whenever I have an image with pure white background, I have to make sure to not use the watermark on export feature in Adobe Photoshop Lightroom in order to preserve the white.
I wish Adobe would finally fix this issue. If I am not mistaken, this bug dates back to at least Lightroom 4 and may actually go as far back as one of the point releases in Lightroom 3.