This is a VERY annoying bug. It happens both in 2.8.1 as well as in 2.9.1.

1)Open some directory full of images using "folders"
2)click on a image to select it
3)press "F" to view it in fullscreen, then "F" again to go back to folders
4)select another image
5)go back to step 3.

randomly, yet very, very often (let's say once every 3-4 attempts), fullscreen will not display the newly selected image, but instead the previous one.

This also happens if you move to a different folder.