On my newly installed Plasma, whenever I want to focus on a window of an app that has multiple instances open, I always need to click at least twice on its icon for the thumbnail view to appear. On the first click the thumbnail view appears and immediately vanishes, and it only stays visible on the >= 2nd try.
The easiest way to explain is through the attached video examples:
https://ufile.io/zbh270b9
https://ufile.io/ds0vucom
If there's a more conventional website for video sharing, let me know and I will edit the post.
