I was reluctant to report this bug as it's extremely rare and I have no idea how to reproduce it on demand. However I've noticed it every now and then, and it has been there for quite a while now.

Sometimes, typically after keeping a Dolphin instance open for a while and having navigated between different directories, previews will stop working properly: If you drag any new files into the current window, previews disappear for all other items and they only show up as their icon. The issue goes away once you press F5 to refresh, but its cause persists in that instance so if you change the contents of the directory again you'll see the same problem.

To replicate: Open a Dolphin instance and try using it for as long as possible to do various activities (I don't yet know the exact trigger). Then navigate to a directory containing images where preview mode is enabled. Drop a new image into or out of this window, and once you've done that you should see that previews disappear and Dolphin acts as if they're disabled. You'll see previews again if you navigate out and back into the directory or press F5, but the issue will persist as you move files around until you restart Dolphin.