Hi Bjorn - the corners go white if the material is in use, yellow if obectts using that material are selected, gray otherwise… The Units sets the scale of the preview - this can be useful to set if there is a texture and not just a color. Set this in the preview thumbnail context menu > Properties.
It is a bug that is difficult to get a working example of. I see it all the time, but can’t seem to create an example file that reliably shows the issue.