On my ASUS laptop with Windows 10 (UX425J) I have a problem very similar to what is described in Thumbnails (only .png) and some icons on taskbar stop showing (Windows 10) and Can't see PNG previews in Windows 7?. Specifically, the .png files are the only ones I cannot see previews on thumbnails. Weirdly, the problem goes away, as I established experimentally, if and only if I set Paint.NET to be the default opening app for PNGs. Of course, that stopgap measure is very inconvenient when actually opening PNGs, so I would prefer being able to have a proper viewer associated without losing thumbnails.
The problem seems to be very persistent; in fact, it was present in the laptop already at the moment of purchase (so I cannot go back to any previous state), and was back then much more pervasive: no image file was having a thumbnail, and some of the apps on the taskbar (such as Store) were having blank icons as well. Simply by installing K-Lite Codec Pack, I managed to fix all the thumbnails and icons - except the persistent PNGs!
The Always show icons, never thumbnails method as given in the Windows 7 question was the thing I tried first, and it never worked; the registry edit method from the Windows 10 one helped for 10 seconds or something, and then the thumbnails promptly vanished once again.
Lastly, the problem was not solved by reinstalling Windows 10 using Media Creation Tool.
Edited to demonstrate how the files look with different default apps
