I always had this bug, at least since I started using Windows 10 when it came out (I am now using version 1709, it's still there).
I think the thumbnail preview for certain transparent icon is broken. Specifically, I am talking about the notepad icon:
a is a folder with an .srt file (in my PC this type of file is opened with notepad, so it automatically gets that icon). I've copied the same file outside the folder to show you it's not a broken icon. Notice the red/green artifact in the preview, where the transparent part of the icon should be.
b is the same, but with two .srt files inside the folder. The icons in the previews are tilted, so the transparent part is hidden.
I've unsuccessfully tried to reproduce this bug with other icons. At most, what I obtained was a partial transparent icon, not coloured artifacts, like in this second image:
So, am I missing something? Some graphic/performance options I do not know of?