I don't think the video update is actually paused when in background, because there are other sw tools on the internet (that I also use) able to grab the screen (i.e. not the stream) of videos running in window that is completely in background.
But in Tiles, Thumbnails are live only if another window that covers the first in not Maximised (i.e. it can also be completely covering the video or be full screen and the thumbnails still live, but if maximise is clicked than the live stops).
So, there must be some way for Tiles to behave like the other tools do. The main benefit of Tiles is to actually see the other windows in preview all the time, so a full-live option would be killer feature