I have also been having this issue for about a week. Thought it may get itself sorted out and I am not on every day. So I didn't think much of it. But since the issue is continuing, I checked here. Glad I'm not alone at least!
Like others with the problem, the Unviewed button is my primary method of viewing the gallery. Perhaps this is only effecting those who use it regularly for some reason?
The main pages of the gallery still show a "NEW" indicator under images I have not seen yet. So whatever keeps track of whether or not I viewed is working. It's something in the compilation of the "Unviewed" page.
I know little to nothing about coding, but is it possible that the system can keep track of whether an image is viewed indefinitely (or up to a very large number), but the method for sorting through these indicators and displaying unviewed images on that tab can only handle up to a certain number of "viewed" images? That would explain why those who use the feature frequently would see an effect while others wouldn't. If this guess is right, anyone who has viewed almost all of the images on the gallery will be having this issue. Tera, can you verify if you or your other previous testers have viewed that many images? Can any others experiencing this problem verify this as well?
No rush for a fix from me either! Just trying to assist in any way I can