AppStore Update progress bar disappears

Recently I upgraded my iMac to Yosemite 10.10.2 and now I have a small problem with the Update page: I clicked on it to install new updates (for iMovie and security updates). The iMovie update is downloading ca. 2 GiB so it takes a while. I closed the AppStore application window and later wanted to see the progress again. I opened the AppStore application again and it showed the progress bar. Ca. 1 second later it shows that it searches for updates again (the round progress indicator at top left position) and then shortly after finishes searching. Ca. 3 seconds later the progress bar of the updating application disappears and it's update button's caption changes from PAUSE to UPDATE, as if the update is not going on anymore (but it does in the background). Clicking on the "resetted" UPDATE button again lets the round progress indicator appear but nothing happens anymore. The page "Purchases" shows the progress bar and button correctly.

I looks like a bug to me. It seems that the AppStore application is showing me the update progress correctly but then it starts to search for new updates again and wrongly resets the interface although the update is still going on. This was not like that in Mavericks.

Example video: https://vimeo.com/122088377