ah, ya, I did some more research and apparently after a certain amount of time of being minimized (maybe) the live preview thumbnail is discarded because it is outdated and replaced by just an icon of the application. then, once you restore the application it is refreshed. I'd rather have an outdated thumbnail than have the icon, though.
The option "save taskbar thumbnail previews" seems to keep the thumbnail, even if its outdated. i'm not sure if this option affects performacne much though, I couldn't find much about it on the internet
thanks!