In Files 3.10.1 (formerly Nautilus) I display thumbnails when viewing a folder of images. If the orientation of some images is wrong (i.e. landscape format displaying as portrait, or vice versa) I rotate the image to correct the orientation, and save the file when I close it. Sometimes the corrected orientation is represented by the thumbnail, but sometimes the thumbnail remains incorrect.
I have deleted ~/.thumbnails/fail
I have run rm -r ~/.thumbnails
I have refreshed using F5
There is another odd symptom, which may help explain the problem to someone. Suppose I have an image that appears as a thumbnail wrongly oriented, rotated 90 degrees clockwise. So I open and rotate the image 90 degree counter-clockwise to correct the orientation. The thumbnail still displays with the wrong orientation, but when I open the image using Files it is correctly oriented. If I open it in GIMP, however, it has the wrong orientation. It seems as if Files has not really rotated the image file, but knows that it should display it rotated when it opens the image. (But GIMP does not know that it should display it this way).