Album thumbnails have vanished

[Elle Stone]

I'm trying to set digiKam up to allow me to keep track of folders with 
editing steps towards final prints. So I set up one album (on-disk 
folder) per final print, and transferred all the relevant files to each 
album.

A couple days ago I spent a fair amount of time clicking on each "final 
print" album and selecting an album thumbnail. It was really cool to see 
these thumbnails - this was a feature I didn't know digiKam had - and 
the visual clue as to the album contents made it just that much easier 
to locate a given set of images.

The next day some of the album thumbnails had disappeared.

This morning all of the album thumbnails are gone.

It is true that I've been rearranging the albums and the contents of the 
albums. But much of the rearranging has been done today, and the album 
thumbnails were already gone when I started digiKam this morning.

Has anyone else had issues with album thumbnails disappearing?

Is there a way to keep the album thumbnails from disappearing?

Is the act of renaming an album, and/or moving images in and out of an 
album, and/or moving the album itself to a different higher-level album, 
supposed to cause the album thumbnail to disappear?

Best,
Elle