I think this is a bug particular to Slideshow > Export Video. I tested with 5800 1000x750 duplicate JPEGs, changing the exposure of the first one to be very dark and then syncing that change to all the others. When I exported all 5800 pics to a folder, they all got the changed exposure; ditto when I did Slideshow > Export PDF. But when I did Slideshow > Export Video, I observed the same symptoms you did: Only those pics for which I had actually viewed thumbnails in Library Grid view got the correct exposure setting in the video.
I recommend that you report this bug in the official Adobe feedback forum, where Adobe has asked that all bugs be reported: Recently active topics in Photoshop Family about Photoshop Lightroom. Be sure to include a step-by-step detailed recipe for reproducing the bug. Adobe tends not to pay attention to reports for which they can't quickly reproduce the symptoms.
Thanks for the response. Good to know others are experiencing the same issue.
I had thought to export all and see if the changes would be reflected in the exported images, but it was taking so long to export that I just canceled it. I just took a sample of about 100 pictures which included some images where the preview image had not reflected the changes and they all exported with the changes successfully.
I was planning on editing the time lapse in Premiere anyway, so I should be good now. Thanks again!