When changing the sorting order from file name to date (or vice versa), the thumbnails are no longer in sync with the displayed images: the position of the thumbnail (in the sense of a link to the images) represents the correct order but the thumbnail image itself, shows a complete unrelated picture. After restarting ristretto the thumbnails are displayed correctly again. I see this issue with ristretto 0.6.3.
Igor referenced this bugreport in commit 812cc0ffb0a686167f4b005e42e22c4156ffe889 Update thumbnail bar on changing the sorting order https://git.xfce.org/apps/ristretto/commit?id=812cc0ffb0a686167f4b005e42e22c4156ffe889