There seems to be an issue with Export function when it is used with a graphic watermark on RAW images. If you export a batch raw images to JPG with a graphic watermarks, some of the output images are blank but has still has a watermark on it.
It doesn't happen when you export two or less images. It is almost like the third parallel operation is causing this issue.
This is a known bug -- see this bug report in the official Adobe feedback forum: LIGHTROOM CC - Random white images when exporting or publishing. See this post in that thread for a slow workaround: LIGHTROOM CC - Random white images when exporting or publishing. Please add your vote and opinion to the topic to help ensure that Adobe will prioritize a fix. If you follow that topic (or reply to it), you'll be notified of status updates, better workarounds, and fixes.