This bug happens when you replace a folder icon with a png-icon which is not belong to the system. As soon as you use the "preview button" and the folder contains at least one file whose view can be modified by this button, such as a jpg image, the new icon will disappear and be replaced by a sheet of paper icon. Reproducible: Always
Thanks for the bug report! I can confirm the problem. However, it's not Dolphin's fault. The code that generates the previews for images and folders is in kde-runtime. I hope that kio/thumbnail is the right product.
Works for me in KDE Frameworks 5.45.