Here is my Lightroom Mobile web album.
I use Lightroom 6, which is up to date. The last photo in the album above is a screen shot of this collection, taken in Lightroom on my Mac. You can see that there are no white areas on the sides of any photos in the collection in Lightroom 6 Mac.
These fisheye photos have been heavily processed in Lightroom, with custom lens profiles and heavy use of the Transform controls. To me it looks like some of the transformations I did in Lightroom on the Mac are not being applied to the images on the web page.
I stopped syncing this gallery from Lightroom on the Mac and manually deleted all of the images from the website. Then I synced again. I see the same things now as I did before. Nothing changed.
I also exported one of the problem images from Lightroom to JPEG, and that works as expected with no problems.
Comments? Suggestions? Is this a bug in Lightroom?
I have an answer to this. What I am seeing on the web is my photos without their custom lens profiles applied. If I use stock profiles instead of custom ones, I won't have this problem. This is now a known problem to the Lightroom team. Thanks to Josh Haftel for explaining this to me.