Dear Maintainer,
In my favorite applications (mate-default-applications-properties), I configured Atril as a document viewer.
In recent days, I have no more thumbnails for PDF. I looked for the cause and I realized that it is Evince which is used for the miniatures and not Atril.
In fact, there is a problem between Evince and Apparmor that blocks the creation of thumbnails, see bug 915024.
When we select Atril as a document viewer in mate-default-applications-properties, I think it should also be used for thumbnails.
Regards.