i found a bug on 1.3.0 if you choose visual thumb selection in admin tab (option/gallery).

in this case, if you have sub-album and if you have choosen random thumbnail for the parent album, you always stay with the most recent picture of sub-albums.

if you unchecked visual thumb selection, and if you have choosen random thumbnail for the parent album, all is going well : you have a random thumb of the sub-album.

Fixed in tonight's build.

tanks !