I can seemingly reproduce it consistently with a single tab open:
- Upgrade an app
- (might need to refresh the page to clear the non-dismissable “upgrade app” dialog)
- Whilst the app is still upgrading, click on “Discover Apps”.
- See all apps shown as “Installed”.
There might be other ways to trigger it, but this is the one I could find.
EDIT: You can also use the “Upgrade All” button. Then, you can skip step 2. The bug still shows.