I'm wondering if this is reproducible and whether it is an openSUSE issue or KDE issue.

Test case:
  1. Populate a folder with many medium to high resolution photos. I used this the photos from this archive:
  2. Configure KDialog for "Short View" with "Show Preview" toggled on
  3. Using web browser that uses KDialog as file picker, navigate to site that gives option of upload in an image e.g. imgur.com
  4. Select the upload link on site to open KDialog
  5. Navigate to folder containing images from step 1
  6. Immediately begin scrolling before all thumbnails have rendered
  7. Many thumbnails will not render unless they are hovered over

This issue is not present for me if I pause a few seconds before I begin scrolling once I navigate to an image-filled folder.