Let's see if I can clarify this. Here's the sequence:
I. I downloaded and installed Sierra, version 10.12, from my El Cap installation via App Store.
2. I launched App Store, which told me that four updates (Pages, Keynote, Numbers, and 10.12.1) were available. I proceeded to download and install all of them. App Store seemed to be behaving normally.
3. I noticed at MacUpdate that there were updates to Pages, Keynote, and Numbers beyond the versions that I previously updated. Consequently, I opened App Store and clicked the Updates tab.
4. App Store showed the updates that I had installed in the last 30 days and the Updates icon had the number 5. BUT, there was no list of which updates were now available and no button(s) to either Update or Update All.
5. I quit App Store, deleted ~/Library/Caches/com.apple.appstore, and relaunched App Store. There was no difference.
6. Since I was aware of which updates were listed (via MacUpdate), I selected the Featured tab of App Store, clicked Apps Made by Apple, and was able to update the apps in question from there (plus iMovie and Garage Band). I went back to the Updates tab, which showed the same as before. Interestingly, the Featured tab, with Apps Made By Apple, had showed which updates were available because they each had an Update button. After installation, the button changed to Open.
7. I had seen, from CNET, that an update to iTunes was available. I launched iTunes, got a message that there was an update, and opted to update. App Store's Updates tab opened and showed the download progress as it should, and indicated when the installation was complete. It then said Checking for Updates and reported that there were none, nor did it list updates installed in the last 30 days.
That's where it stands for now.
