"Stick to what you know - it's a SERVER SIDE bug where it is serving an
incorrect page for updates. There have been ZERO changes to the 10.13
app store in years."
Agree, it's an issue with Apple's Servers. I believe they've just pulled the plug on the old method of serving the formatting for this App Store update page, because new versions of macOS don't use this method to show the updates available, and with High Sierra out of support, some bright spark said, well we don't need this now,
With companies the size of Apple, their backend servers can have many forms and functions, that break down bigger processes into smaller more manageable server processes.
To those saying it's an local hardware/software issue. OK, so me a machine running High Sierra that doesn't exhibit this issue and I'll accept that, but I have access to a range of iMacs / macbooks running High Sierra and I'm yet to see one running High Sierra that doesn't exhibit this issue. Can't put it more plainly than that.
Yes, it's quite subtle what Apple has done here. It's clearly not the server that serves the updates themselves. It's not the server that generates the text in different languages.
It's the server that generates the formatted css/javascript page to show the updates available for a specific device request. What we don't is the 'secret sauce' being used here by Apple, in what checks it makes in order to serve that page. How Apple indexes all this their end, keeps track of what updates have been applied.
To anyone that says it's not an Apple formatting server problem show me a 2017 macbook pro (Sept 2018 purchased pre-installed with High Sierra) or any Apple device running High Sierra that doesn't exhibit this problem.
What I'd recommend:
If you have an Apple Store close by, book an appointment by phone with a Apple Genius, say that your issue is related to High Sierra and ask if they have a machine running High Sierra that can be used so you can explain the issue.
Once they bring out the machine running High Sierra, go straight to the App Store Update Page on the Apple Store machine, and the fault will be there, because of all the macs we have access to, various years and devices, I'm yet to see a machine running High Sierra that hasn't exhibited this fault since the change that happened April 4th.